Phase 1: Add pgqueuer infrastructure and worker skeleton

- Add pgqueuer dependency to pyproject.toml
- Create worker.py with basic setup:
  - Independent database connection using asyncpg
  - Install pgqueuer schema on startup
  - Register dummy job handler
  - Start consumer loop
- Add 'make worker' command
- Update 'make dev' to run worker alongside backend/frontend

Validation:
- Worker starts successfully
- pgqueuer tables exist in database
- All existing tests pass

backend/pyproject.toml

@@ -11,6 +11,7 @@ dependencies = [
     "python-jose[cryptography]>=3.3.0",
     "email-validator>=2.0.0",
     "bech32>=1.2.0",
+    "pgqueuer>=0.1.0",
 ]
 
 [dependency-groups]