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

.dockerignore

@@ -1,5 +1,4 @@
 .git
-__pycache__
-*.pyc
-.env
-bellhop.db
+data/
+config.yaml
+bellhop