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>

apps/coordinator/Dockerfile

@@ -0,0 +1,59 @@
+# Multi-stage build for mosaic-coordinator
+FROM python:3.11-slim AS builder
+
+WORKDIR /app
+
+# Install build dependencies
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+    build-essential \
+    && rm -rf /var/lib/apt/lists/*
+
+# Copy dependency files
+COPY pyproject.toml .
+
+# Create virtual environment and install dependencies
+RUN python -m venv /opt/venv
+ENV PATH="/opt/venv/bin:$PATH"
+RUN pip install --no-cache-dir --upgrade pip && \
+    pip install --no-cache-dir hatchling && \
+    pip install --no-cache-dir \
+    fastapi>=0.109.0 \
+    uvicorn[standard]>=0.27.0 \
+    pydantic>=2.5.0 \
+    pydantic-settings>=2.1.0 \
+    python-dotenv>=1.0.0
+
+# Production stage
+FROM python:3.11-slim
+
+WORKDIR /app
+
+# Copy virtual environment from builder
+COPY --from=builder /opt/venv /opt/venv
+ENV PATH="/opt/venv/bin:$PATH"
+
+# Copy application code
+COPY src/ ./src/
+
+# Create non-root user
+RUN useradd -m -u 1000 coordinator && \
+    chown -R coordinator:coordinator /app
+
+USER coordinator
+
+# Environment variables
+ENV PYTHONUNBUFFERED=1 \
+    PYTHONDONTWRITEBYTECODE=1 \
+    HOST=0.0.0.0 \
+    PORT=8000
+
+# Health check
+HEALTHCHECK --interval=30s --timeout=10s --start-period=5s --retries=3 \
+    CMD python -c "import urllib.request; urllib.request.urlopen('http://localhost:8000/health')"
+
+# Expose port
+EXPOSE 8000
+
+# Run application
+CMD ["uvicorn", "src.main:app", "--host", "0.0.0.0", "--port", "8000"]