build: add Dockerfile and docker compose stack

[wowi42]

Add Docker packaging for the full stack.

• Dockerfile on python:3.14.7-slim with uv 0.12.7
• docker-compose.yml with three app services (api, consume, mcp) plus postgres 18.6 and rabbitmq 4.3-management, sharing env via a YAML anchor (DATABASE_URL, CELERY_BROKER_URL, API_KEY, LOG_LEVEL)
• Move httpx to main dependencies: alerting and MCP tools import it at runtime
• MCP tools no longer send a Bearer header when API_KEY is empty
• MCP service points at the api service via TASKOWL_HOST

Verified: image builds, all five containers start, /health returns ok, and an MCP tools/call reaches the API.