Remove Google Sheets mirror entirely (#13)

Postgres is now the sole source of truth: delete sheets.py and
import_from_sheets.py, strip mirror_entity/mirror_async/_mirror_worker and
POST /api/sync from app.py, drop the tab/mirror keys from db.py's TABLES.
Re-point n8n/renewal-reminder.json at the CRM's own HTTP API (GET
/api/clients, POST /api/activity_log) instead of the Sheets nodes, and drop
SHEET_ID/GOOGLE_SA_JSON from deploy env/compose and requests from
requirements.txt (PyJWT stays — still used by booking_api.py). Updates
docs/README/playbooks accordingly and closes the old #5 (atomic mirror) as
moot.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backoffice/app/requirements.txt

@@ -8,7 +8,6 @@ Flask==3.0.3
 psycopg[binary]==3.2.1
 waitress==3.0.0
 PyJWT[crypto]==2.9.0
-requests==2.32.3
 # python:3.12-slim has no system IANA tz database; zoneinfo (used by
 # availability.py for Europe/Berlin) falls back to this package for it.
 tzdata==2024.1