feat: initial raven-notify — Discord + email alerts via POST /notify

FastAPI at port 8400. Both channels fire on every notification.
Grafana webhook endpoint ready for later wiring.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

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+FROM python:3.12-slim
+WORKDIR /app
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+COPY main.py .
+CMD ["python", "main.py"]