feat: Add E2E encryption support to Matrix bot

- matrix-nio[e2e] with libolm for Megolm encryption
- Persistent crypto store volume for key persistence
- Auto-accept key verification (SAS)
- Upload device keys on first login

CF-1147

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

Dockerfile

@@ -1,6 +1,6 @@
 FROM python:3.11-slim
 WORKDIR /app
-RUN apt-get update && apt-get install -y --no-install-recommends ffmpeg && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y --no-install-recommends ffmpeg libolm-dev && rm -rf /var/lib/apt/lists/*
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 COPY . .