feat(#157): Set up webhook receiver endpoint

Implement FastAPI webhook receiver for Gitea issue assignment events
with HMAC SHA256 signature verification and event routing.

Implementation details:
- FastAPI application with /webhook/gitea POST endpoint
- HMAC SHA256 signature verification in security.py
- Event routing for assigned, unassigned, closed actions
- Comprehensive logging for all webhook events
- Health check endpoint at /health
- Docker containerization with health checks
- 91% test coverage (exceeds 85% requirement)

TDD workflow followed:
- Wrote 16 tests first (RED phase)
- Implemented features to pass tests (GREEN phase)
- All tests passing with 91% coverage
- Type checking with mypy: success
- Linting with ruff: success

Files created:
- apps/coordinator/src/main.py - FastAPI application
- apps/coordinator/src/webhook.py - Webhook handlers
- apps/coordinator/src/security.py - HMAC verification
- apps/coordinator/src/config.py - Configuration management
- apps/coordinator/tests/ - Comprehensive test suite
- apps/coordinator/Dockerfile - Production container
- apps/coordinator/pyproject.toml - Python project config

Configuration:
- Updated .env.example with GITEA_WEBHOOK_SECRET
- Updated docker-compose.yml with coordinator service

Testing:
- 16 unit and integration tests
- Security tests for signature verification
- Event handler tests for all supported actions
- Health check endpoint tests
- All tests passing with 91% coverage

This unblocks issue #158 (issue parser).

Fixes #157

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docker/docker-compose.yml

@@ -39,6 +39,35 @@ services:
     networks:
       - mosaic-network
 
+  coordinator:
+    build:
+      context: ../apps/coordinator
+      dockerfile: Dockerfile
+    container_name: mosaic-coordinator
+    restart: unless-stopped
+    environment:
+      GITEA_WEBHOOK_SECRET: ${GITEA_WEBHOOK_SECRET}
+      GITEA_URL: ${GITEA_URL:-https://git.mosaicstack.dev}
+      LOG_LEVEL: ${LOG_LEVEL:-info}
+      HOST: 0.0.0.0
+      PORT: 8000
+    ports:
+      - "8000:8000"
+    healthcheck:
+      test:
+        [
+          "CMD",
+          "python",
+          "-c",
+          "import urllib.request; urllib.request.urlopen('http://localhost:8000/health')",
+        ]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 5s
+    networks:
+      - mosaic-network
+
 volumes:
   postgres_data:
     name: mosaic-postgres-data