feat(e2ee): Add HKDF E2EE support for Element Call compatibility

Element Call uses HKDF-SHA256 + AES-128-GCM for frame encryption,
while the LiveKit Rust SDK defaults to PBKDF2 + AES-256-GCM.

- Multi-stage Dockerfile builds patched Rust FFI from EC-compat fork
- Generates Python protobuf bindings with new fields
- patch_sdk.py modifies installed livekit-rtc for new proto fields
- agent.py passes E2EE options with HKDF to ctx.connect()
- bot.py exchanges encryption keys via Matrix state events
- Separate Dockerfile.bot for bot service (no Rust build needed)

Ref: livekit/rust-sdks#904, livekit/python-sdks#570

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

Dockerfile.bot

@@ -0,0 +1,6 @@
+FROM python:3.11-slim-bookworm
+WORKDIR /app
+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 . .