feat(internal): add NATS abstraction and DB connection manager

- internal/nats: Connection interface + InProcessConn, 7 tests
- internal/db: PostgreSQL connection pool + migration runner, 5 tests
- 75 total tests, build + vet clean

backend/go.mod

@@ -3,3 +3,5 @@ module git.yeij.top/AskaEth/Cyrene_v2
 go 1.25.0
 
 require github.com/joho/godotenv v1.5.1
+
+require github.com/lib/pq v1.12.3