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
5a706fedc4