Harden API security: input validation, safe auth extraction, new tests, and deploy config

Comprehensive security hardening from audit findings:
- Add validation tags to all DTO request structs (max lengths, ranges, enums)
- Replace unsafe type assertions with MustGetAuthUser helper across all handlers
- Remove query-param token auth from admin middleware (prevents URL token leakage)
- Add request validation calls in handlers that were missing c.Validate()
- Remove goroutines in handlers (timezone update now synchronous)
- Add sanitize middleware and path traversal protection (path_utils)
- Stop resetting admin passwords on migration restart
- Warn on well-known default SECRET_KEY
- Add ~30 new test files covering security regressions, auth safety, repos, and services
- Add deploy/ config, audit digests, and AUDIT_FINDINGS documentation

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

internal/monitoring/service.go

@@ -108,6 +108,10 @@ func (s *Service) Stop() {
 	close(s.settingsStopCh)
 
 	s.collector.Stop()
+
+	// Flush and close the log writer's background goroutine
+	s.logWriter.Close()
+
 	log.Info().Str("process", s.process).Msg("Monitoring service stopped")
 }