feat: add Gitea as webhook-based service provider

First webhook-based provider integration (Immich uses polling).
Gitea pushes events via POST /api/webhooks/gitea/{provider_id} with
HMAC-SHA256 signature validation.

- 9 event types: push, issue opened/closed/commented, PR opened/closed/merged/commented, release published
- Generic filters system on NotificationTracker (collections, senders, exclude_senders)
- Provider capabilities include supported_filters and webhook_based flag
- Gitea API client for connection testing and repository listing
- 18 default Jinja2 notification templates (EN + RU)
- Frontend: conditional provider forms, Gitea event toggles in tracking config
- Auto-migration for filters column and Gitea tracking flags

frontend/src/lib/grid-items.ts

@@ -99,4 +99,5 @@ export const previewTargetTypeItems = (): GridItem[] => [
 
 export const providerTypeItems = (): GridItem[] => [
 	{ value: 'immich', icon: 'mdiCamera', label: t('providers.typeImmich') },
+	{ value: 'gitea', icon: 'mdiGit', label: t('providers.typeGitea') },
 ];