Phase 1: Add pgqueuer infrastructure

- Add pgqueuer dependency to pyproject.toml
- Create worker.py with schema installation and job handler registration
- Add make worker command to Makefile
- Update make dev to run worker alongside backend/frontend
- Use has_table() check for idempotent schema installation
- Register 'random_number' job handler (placeholder that logs for now)

Makefile

@@ -48,8 +48,8 @@ db-seed: db-ready
 dev:
 	$(MAKE) db-seed
 	cd backend && uv run uvicorn main:app --reload & \
-	cd frontend && npm run dev & \
 	cd backend && uv run python worker.py & \
+	cd frontend && npm run dev & \
 	wait
 
 # TEST variable can be used to select specific tests: