feat: store layer with indexes, optimistic concurrency, GridFS, credential crypto (phase 2)

- mongo-driver v2 connection with bounded startup retry + /readyz gate
- idempotent creation of all §4.9 indexes
- UpdateVersioned: version-filtered updates, conflict vs not-found errors
- AES-256-GCM Seal/Open for ticketing credentials, base64(nonce|ct)
- GridFS file store: MIME sniffing, MAX_UPLOAD_MB cap, sha256 metadata
- HMAC-signed short-lived file URL tokens (§5.1, 1h TTL)
- integration tests against compose Mongo via docker-compose.test.yml overlay

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Makefile

@@ -11,6 +11,8 @@ run:
 test:
 	$(GO) test ./...
 
+# Requires Mongo reachable on 127.0.0.1:27017 (or MONGO_TEST_URI):
+#   docker compose -f docker-compose.yml -f docker-compose.test.yml up -d mongo
 test-integration:
 	$(GO) test -tags=integration ./...