feat: add GitHub and GitLab webhook support (#170)

## Summary

Adds GitHub and GitLab push webhook support alongside the existing Gitea support.

closes #68

## What Changed

### Auto-detection of webhook source

The webhook handler now auto-detects which platform sent the webhook by examining HTTP headers:
- **Gitea**: `X-Gitea-Event`
- **GitHub**: `X-GitHub-Event`
- **GitLab**: `X-Gitlab-Event`

Existing Gitea webhooks continue to work unchanged. Unknown sources fall back to Gitea format for backward compatibility.

### Normalized push event

All three payload formats are parsed into a unified `PushEvent` struct containing:
- Source platform, ref, branch, commit SHA
- Repository name, clone URL, HTML URL
- Commit URL (with per-platform fallback logic)
- Pusher username/name

### New files

- **`internal/service/webhook/payloads.go`**: Source-specific payload structs (`GiteaPushPayload`, `GitHubPushPayload`, `GitLabPushPayload`), `ParsePushPayload()` dispatcher, per-platform parsers, branch extraction, and commit URL extraction functions.

### Modified files

- **`internal/service/webhook/types.go`**: Added `Source` type (gitea/github/gitlab/unknown), `DetectWebhookSource()`, `DetectEventType()`, and `PushEvent` normalized type. Moved `GiteaPushPayload` to payloads.go.
- **`internal/service/webhook/webhook.go`**: `HandleWebhook` now accepts a `Source` parameter and uses `ParsePushPayload()` for unified parsing instead of directly unmarshaling Gitea payloads.
- **`internal/handlers/webhook.go`**: Calls `DetectWebhookSource()` and `DetectEventType()` to auto-detect the platform before delegating to the webhook service.
- **`internal/service/webhook/webhook_test.go`**: Comprehensive tests for source detection, event type extraction, payload parsing (all 3 platforms), commit URL fallback paths, and integration tests through `HandleWebhook` for GitHub and GitLab sources.
- **`README.md`**: Updated description, features, non-goals, and architecture to reflect multi-platform webhook support.

## Test coverage

Webhook package: **96.9%** statement coverage. Tests cover:
- `DetectWebhookSource` with all header combinations and precedence
- `DetectEventType` for each platform
- `ParsePushPayload` for Gitea, GitHub, GitLab, unknown source, invalid JSON, empty payloads
- Commit URL extraction fallback paths for GitHub and GitLab
- Direct struct deserialization for all three payload types
- Full `HandleWebhook` integration tests with GitHub and GitLab sources

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #170
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/service/webhook/payloads.go

@@ -0,0 +1,248 @@
+package webhook
+
+import "encoding/json"
+
+// GiteaPushPayload represents a Gitea push webhook payload.
+//
+//nolint:tagliatelle // Field names match Gitea API (snake_case)
+type GiteaPushPayload struct {
+	Ref        string      `json:"ref"`
+	Before     string      `json:"before"`
+	After      string      `json:"after"`
+	CompareURL UnparsedURL `json:"compare_url"`
+	Repository struct {
+		FullName string      `json:"full_name"`
+		CloneURL UnparsedURL `json:"clone_url"`
+		SSHURL   string      `json:"ssh_url"`
+		HTMLURL  UnparsedURL `json:"html_url"`
+	} `json:"repository"`
+	Pusher struct {
+		Username string `json:"username"`
+		Email    string `json:"email"`
+	} `json:"pusher"`
+	Commits []struct {
+		ID      string      `json:"id"`
+		URL     UnparsedURL `json:"url"`
+		Message string      `json:"message"`
+		Author  struct {
+			Name  string `json:"name"`
+			Email string `json:"email"`
+		} `json:"author"`
+	} `json:"commits"`
+}
+
+// GitHubPushPayload represents a GitHub push webhook payload.
+//
+//nolint:tagliatelle // Field names match GitHub API (snake_case)
+type GitHubPushPayload struct {
+	Ref        string `json:"ref"`
+	Before     string `json:"before"`
+	After      string `json:"after"`
+	CompareURL string `json:"compare"`
+	Repository struct {
+		FullName string      `json:"full_name"`
+		CloneURL UnparsedURL `json:"clone_url"`
+		SSHURL   string      `json:"ssh_url"`
+		HTMLURL  UnparsedURL `json:"html_url"`
+	} `json:"repository"`
+	Pusher struct {
+		Name  string `json:"name"`
+		Email string `json:"email"`
+	} `json:"pusher"`
+	HeadCommit *struct {
+		ID      string      `json:"id"`
+		URL     UnparsedURL `json:"url"`
+		Message string      `json:"message"`
+	} `json:"head_commit"`
+	Commits []struct {
+		ID      string      `json:"id"`
+		URL     UnparsedURL `json:"url"`
+		Message string      `json:"message"`
+		Author  struct {
+			Name  string `json:"name"`
+			Email string `json:"email"`
+		} `json:"author"`
+	} `json:"commits"`
+}
+
+// GitLabPushPayload represents a GitLab push webhook payload.
+//
+//nolint:tagliatelle // Field names match GitLab API (snake_case)
+type GitLabPushPayload struct {
+	Ref       string `json:"ref"`
+	Before    string `json:"before"`
+	After     string `json:"after"`
+	UserName  string `json:"user_name"`
+	UserEmail string `json:"user_email"`
+	Project   struct {
+		PathWithNamespace string      `json:"path_with_namespace"`
+		GitHTTPURL        UnparsedURL `json:"git_http_url"`
+		GitSSHURL         string      `json:"git_ssh_url"`
+		WebURL            UnparsedURL `json:"web_url"`
+	} `json:"project"`
+	Commits []struct {
+		ID      string      `json:"id"`
+		URL     UnparsedURL `json:"url"`
+		Message string      `json:"message"`
+		Author  struct {
+			Name  string `json:"name"`
+			Email string `json:"email"`
+		} `json:"author"`
+	} `json:"commits"`
+}
+
+// ParsePushPayload parses a raw webhook payload into a normalized PushEvent
+// based on the detected webhook source. Returns an error if JSON unmarshaling
+// fails. For SourceUnknown, falls back to Gitea format for backward
+// compatibility.
+func ParsePushPayload(source Source, payload []byte) (*PushEvent, error) {
+	switch source {
+	case SourceGitHub:
+		return parseGitHubPush(payload)
+	case SourceGitLab:
+		return parseGitLabPush(payload)
+	case SourceGitea, SourceUnknown:
+		// Gitea and unknown both use Gitea format for backward compatibility.
+		return parseGiteaPush(payload)
+	}
+
+	// Unreachable for known source values, but satisfies exhaustive checker.
+	return parseGiteaPush(payload)
+}
+
+func parseGiteaPush(payload []byte) (*PushEvent, error) {
+	var p GiteaPushPayload
+
+	unmarshalErr := json.Unmarshal(payload, &p)
+	if unmarshalErr != nil {
+		return nil, unmarshalErr
+	}
+
+	commitURL := extractGiteaCommitURL(p)
+
+	return &PushEvent{
+		Source:    SourceGitea,
+		Ref:       p.Ref,
+		Before:    p.Before,
+		After:     p.After,
+		Branch:    extractBranch(p.Ref),
+		RepoName:  p.Repository.FullName,
+		CloneURL:  p.Repository.CloneURL,
+		HTMLURL:   p.Repository.HTMLURL,
+		CommitURL: commitURL,
+		Pusher:    p.Pusher.Username,
+	}, nil
+}
+
+func parseGitHubPush(payload []byte) (*PushEvent, error) {
+	var p GitHubPushPayload
+
+	unmarshalErr := json.Unmarshal(payload, &p)
+	if unmarshalErr != nil {
+		return nil, unmarshalErr
+	}
+
+	commitURL := extractGitHubCommitURL(p)
+
+	return &PushEvent{
+		Source:    SourceGitHub,
+		Ref:       p.Ref,
+		Before:    p.Before,
+		After:     p.After,
+		Branch:    extractBranch(p.Ref),
+		RepoName:  p.Repository.FullName,
+		CloneURL:  p.Repository.CloneURL,
+		HTMLURL:   p.Repository.HTMLURL,
+		CommitURL: commitURL,
+		Pusher:    p.Pusher.Name,
+	}, nil
+}
+
+func parseGitLabPush(payload []byte) (*PushEvent, error) {
+	var p GitLabPushPayload
+
+	unmarshalErr := json.Unmarshal(payload, &p)
+	if unmarshalErr != nil {
+		return nil, unmarshalErr
+	}
+
+	commitURL := extractGitLabCommitURL(p)
+
+	return &PushEvent{
+		Source:    SourceGitLab,
+		Ref:       p.Ref,
+		Before:    p.Before,
+		After:     p.After,
+		Branch:    extractBranch(p.Ref),
+		RepoName:  p.Project.PathWithNamespace,
+		CloneURL:  p.Project.GitHTTPURL,
+		HTMLURL:   p.Project.WebURL,
+		CommitURL: commitURL,
+		Pusher:    p.UserName,
+	}, nil
+}
+
+// extractBranch extracts the branch name from a git ref.
+func extractBranch(ref string) string {
+	// refs/heads/main -> main
+	const prefix = "refs/heads/"
+
+	if len(ref) >= len(prefix) && ref[:len(prefix)] == prefix {
+		return ref[len(prefix):]
+	}
+
+	return ref
+}
+
+// extractGiteaCommitURL extracts the commit URL from a Gitea push payload.
+// Prefers the URL from the head commit, falls back to constructing from repo URL.
+func extractGiteaCommitURL(payload GiteaPushPayload) UnparsedURL {
+	for _, commit := range payload.Commits {
+		if commit.ID == payload.After && commit.URL != "" {
+			return commit.URL
+		}
+	}
+
+	if payload.Repository.HTMLURL != "" && payload.After != "" {
+		return UnparsedURL(payload.Repository.HTMLURL.String() + "/commit/" + payload.After)
+	}
+
+	return ""
+}
+
+// extractGitHubCommitURL extracts the commit URL from a GitHub push payload.
+// Prefers head_commit.url, then searches commits, then constructs from repo URL.
+func extractGitHubCommitURL(payload GitHubPushPayload) UnparsedURL {
+	if payload.HeadCommit != nil && payload.HeadCommit.URL != "" {
+		return payload.HeadCommit.URL
+	}
+
+	for _, commit := range payload.Commits {
+		if commit.ID == payload.After && commit.URL != "" {
+			return commit.URL
+		}
+	}
+
+	if payload.Repository.HTMLURL != "" && payload.After != "" {
+		return UnparsedURL(payload.Repository.HTMLURL.String() + "/commit/" + payload.After)
+	}
+
+	return ""
+}
+
+// extractGitLabCommitURL extracts the commit URL from a GitLab push payload.
+// Prefers commit URL from the commits list, falls back to constructing from
+// project web URL.
+func extractGitLabCommitURL(payload GitLabPushPayload) UnparsedURL {
+	for _, commit := range payload.Commits {
+		if commit.ID == payload.After && commit.URL != "" {
+			return commit.URL
+		}
+	}
+
+	if payload.Project.WebURL != "" && payload.After != "" {
+		return UnparsedURL(payload.Project.WebURL.String() + "/-/commit/" + payload.After)
+	}
+
+	return ""
+}