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>

README.md

@@ -1,12 +1,12 @@
 # µPaaS by [@sneak](https://sneak.berlin)
 
-A simple self-hosted PaaS that auto-deploys Docker containers from Git repositories via Gitea webhooks.
+A simple self-hosted PaaS that auto-deploys Docker containers from Git repositories via webhooks from Gitea, GitHub, or GitLab.
 
 ## Features
 
 - Single admin user with argon2id password hashing
 - Per-app SSH keypairs for read-only deploy keys
-- Per-app UUID-based webhook URLs for Gitea integration
+- Per-app UUID-based webhook URLs with auto-detection of Gitea, GitHub, and GitLab
 - Branch filtering - only deploy on configured branch changes
 - Environment variables, labels, and volume mounts per app
 - CPU and memory resource limits per app
@@ -20,7 +20,7 @@ A simple self-hosted PaaS that auto-deploys Docker containers from Git repositor
 - Complex CI pipelines
 - Multiple container orchestration
 - SPA/API-first design
-- Support for non-Gitea webhooks
+- Support for non-push webhook events (e.g. issues, merge requests)
 
 ## Architecture
 
@@ -45,7 +45,7 @@ upaas/
 │   │   ├── auth/        # Authentication service
 │   │   ├── deploy/      # Deployment orchestration
 │   │   ├── notify/      # Notifications (ntfy, Slack)
-│   │   └── webhook/     # Gitea webhook processing
+│   │   └── webhook/     # Webhook processing (Gitea, GitHub, GitLab)
 │   └── ssh/             # SSH key generation
 ├── static/              # Embedded CSS/JS assets
 └── templates/           # Embedded HTML templates