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>

deploy/registry.env.example

@@ -0,0 +1,11 @@
+# Container registry used for deploy images.
+# For GHCR:
+#   REGISTRY=ghcr.io
+#   REGISTRY_NAMESPACE=<github-username-or-org>
+#   REGISTRY_USERNAME=<github-username>
+#   REGISTRY_TOKEN=<github-pat-with-read:packages,write:packages>
+
+REGISTRY=ghcr.io
+REGISTRY_NAMESPACE=CHANGEME_NAMESPACE
+REGISTRY_USERNAME=CHANGEME_USERNAME
+REGISTRY_TOKEN=CHANGEME_TOKEN