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/push/fcm.go

@@ -117,6 +117,9 @@ func (c *FCMClient) Send(ctx context.Context, tokens []string, title, message st
 
 	// Log individual results
 	for i, result := range fcmResp.Results {
+		if i >= len(tokens) {
+			break
+		}
 		if result.Error != "" {
 			log.Error().
 				Str("token", truncateToken(tokens[i])).