feat: static sites feature with Gitea/GitHub/GitLab support and Deno backend

Deploy static content from Git repository folders with optional server-side
API endpoints. Supports Gitea/Forgejo/Gogs, GitHub, and GitLab with provider
autodetection.

- New Sites entity with CRUD, encrypted secrets, and manual/push/tag sync triggers
- Pluggable GitProvider interface with three implementations
- Deno container mode: auto-generates router from API_{method}_{name} exports
- Static container mode: nginx serving files with optional markdown rendering
- Wizard UI with provider selector, repo picker, branch/folder tree pickers
- Deploy pipeline builds fresh image, starts container, configures NPM proxy
- Stop/Start buttons, force redeploy on manual trigger
- Periodic health checker detects crashed containers
- Proxy route existence check during auto-sync

internal/proxy/provider.go

@@ -27,6 +27,10 @@ type Provider interface {
 	// Does nothing if routeID is empty.
 	DeleteRoute(ctx context.Context, routeID string) error
 
+	// RouteExists returns true if a proxy route exists for the given domain.
+	// Used for health checks during auto-sync to detect externally removed routes.
+	RouteExists(ctx context.Context, domain string) (bool, error)
+
 	// ContainerLabels returns Docker labels to set on containers at creation time.
 	// Traefik uses labels for auto-discovery; NPM and None return nil.
 	ContainerLabels(domain string, port int) map[string]string