feat: add GitHub and GitLab webhook support

Add auto-detection of webhook source (Gitea, GitHub, GitLab) by
examining HTTP headers (X-Gitea-Event, X-GitHub-Event, X-Gitlab-Event).

Parse push webhook payloads from all three platforms into a normalized
PushEvent type for unified processing. Each platform's payload format
is handled by dedicated parser functions with correct field mapping
and commit URL extraction.

The webhook handler now detects the source automatically — existing
Gitea webhooks continue to work unchanged, while GitHub and GitLab
webhooks are parsed with their respective payload formats.

Includes comprehensive tests for source detection, event type
extraction, payload parsing for all three platforms, commit URL
fallback logic, and integration tests via HandleWebhook.

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
 - Docker builds via socket access
@@ -19,7 +19,7 @@ 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
 
@@ -44,7 +44,7 @@ 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