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/handlers/webhook.go

@@ -7,12 +7,14 @@ import (
 	"github.com/go-chi/chi/v5"
 
 	"sneak.berlin/go/upaas/internal/models"
+	"sneak.berlin/go/upaas/internal/service/webhook"
 )
 
 // maxWebhookBodySize is the maximum allowed size of a webhook request body (1MB).
 const maxWebhookBodySize = 1 << 20
 
-// HandleWebhook handles incoming Gitea webhooks.
+// HandleWebhook handles incoming webhooks from Gitea, GitHub, or GitLab.
+// The webhook source is auto-detected from HTTP headers.
 func (h *Handlers) HandleWebhook() http.HandlerFunc {
 	return func(writer http.ResponseWriter, request *http.Request) {
 		secret := chi.URLParam(request, "secret")
@@ -50,16 +52,17 @@ func (h *Handlers) HandleWebhook() http.HandlerFunc {
 			return
 		}
 
-		// Get event type from header
-		eventType := request.Header.Get("X-Gitea-Event")
-		if eventType == "" {
-			eventType = "push"
-		}
+		// Auto-detect webhook source from headers
+		source := webhook.DetectWebhookSource(request.Header)
+
+		// Extract event type based on detected source
+		eventType := webhook.DetectEventType(request.Header, source)
 
 		// Process webhook
 		webhookErr := h.webhook.HandleWebhook(
 			request.Context(),
 			application,
+			source,
 			eventType,
 			body,
 		)