Add production hardening: health check, streamer panic fix, db maintenance

- Add health check endpoint at /.well-known/healthcheck.json that
  verifies database and RIS Live connectivity, returns 200/503

- Fix panic in streamer when encountering unknown RIS message types
  by logging a warning and continuing instead of crashing

- Add DBMaintainer for periodic database maintenance:
  - VACUUM every 6 hours to reclaim space
  - ANALYZE every hour to update query statistics
  - Graceful shutdown support

- Add Vacuum() and Analyze() methods to database interface

internal/server/routes.go

@@ -24,6 +24,7 @@ func (s *Server) setupRoutes() {
 	r.Get("/", s.handleRoot())
 	r.Get("/status", s.handleStatusHTML())
 	r.Get("/status.json", JSONValidationMiddleware(s.handleStatusJSON()).ServeHTTP)
+	r.Get("/.well-known/healthcheck.json", JSONValidationMiddleware(s.handleHealthCheck()).ServeHTTP)
 
 	// AS and prefix detail pages
 	r.Get("/as/{asn}", s.handleASDetail())