feat: Implement Matrix Rooms Bridge

- MATRIX_ROOMS_BRIDGE_SPEC.md documentation
- Migration 012: Add matrix_room_id/alias to city_rooms
- Matrix Gateway service (port 7025)
- City-service: auto-create Matrix rooms on room creation
- Backfill endpoint for existing rooms
- API returns matrix_room_id/alias in room responses

services/matrix-gateway/Dockerfile

@@ -0,0 +1,17 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+# Install dependencies
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+# Copy application
+COPY . .
+
+# Expose port
+EXPOSE 7025
+
+# Run
+CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "7025"]
+