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>

.env.example

@@ -158,6 +158,11 @@ GITEA_BOT_PASSWORD=REPLACE_WITH_COORDINATOR_BOT_PASSWORD
 GITEA_REPO_OWNER=mosaic
 GITEA_REPO_NAME=stack
 
+# Webhook secret for coordinator (HMAC SHA256 signature verification)
+# SECURITY: Generate random secret with: openssl rand -hex 32
+# Configure in Gitea: Repository Settings → Webhooks → Add Webhook
+GITEA_WEBHOOK_SECRET=REPLACE_WITH_RANDOM_WEBHOOK_SECRET
+
 # ======================
 # Logging & Debugging
 # ======================