Begin Go rewrite: remove Python, scaffold module and config

Pivot from FastAPI web portal to a Matrix command bot (modeled on Pete).
Users will issue !movie / !tv / !music commands in allowlisted rooms; the
bot performs a top-hit search against Radarr/Sonarr/Lidarr and adds it.

This commit is session 1 of a multi-session rewrite (see SESSION_PLAN.md):
  - Delete app/, requirements.txt, old Dockerfile, .env.example
  - Add go.mod (mautrix-go + yaml.v3)
  - Add internal/config: YAML loader with ${ENV} expansion, validates
    matrix creds, allowed_rooms, and per-service *arr config
  - Reset .gitignore / .dockerignore for the Go layout

Dockerfile

@@ -1,12 +0,0 @@
-FROM python:3.12-slim
-
-WORKDIR /app
-
-COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
-
-COPY . .
-
-EXPOSE 8000
-
-CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]