feat(PROJ-13): OpenAPI 3.0 Spec + GET /api/v1/docs Endpoint

Serves the static OpenAPI YAML via go:embed. Completes the last
open acceptance criterion for PROJ-13. PROJ-44 marked Deployed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/server.go

@@ -296,6 +296,7 @@ func (s *Server) routes() {
 	s.mux.HandleFunc("POST /api/admin/users/{id}/totp/reset", s.authAdmin(s.handleTOTPReset))
 
 	// PROJ-13: External REST API v1 (API-key auth)
+	s.mux.HandleFunc("GET /api/v1/docs", s.handleV1Docs)
 	s.mux.HandleFunc("/api/v1/mails", s.apiKeyMw.Wrap(s.handleV1SearchMails))
 	s.mux.HandleFunc("GET /api/v1/mails/{message_id}", s.apiKeyMw.Wrap(s.handleV1GetMail))
 	s.mux.HandleFunc("GET /api/v1/mails/{message_id}/raw", s.apiKeyMw.Wrap(s.handleV1GetMailRaw))