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>

go.mod

@@ -1,3 +1,16 @@
 module bountyboard
 
 go 1.26
+
+require go.mongodb.org/mongo-driver/v2 v2.6.0
+
+require (
+	github.com/klauspost/compress v1.17.6 // indirect
+	github.com/xdg-go/pbkdf2 v1.0.0 // indirect
+	github.com/xdg-go/scram v1.2.0 // indirect
+	github.com/xdg-go/stringprep v1.0.4 // indirect
+	github.com/youmark/pkcs8 v0.0.0-20240726163527-a2c0da244d78 // indirect
+	golang.org/x/crypto v0.33.0 // indirect
+	golang.org/x/sync v0.11.0 // indirect
+	golang.org/x/text v0.22.0 // indirect
+)