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/.env.example

@@ -5,7 +5,6 @@ BOOKING_TOKEN_SECRET=change-me-long-random-too
 # Owner-login session cookie signing key (#19). Dedicated secret -- rotating
 # it just logs owners out, without touching CRM_API_TOKEN/BOOKING_TOKEN_SECRET.
 SESSION_SECRET_KEY=change-me-long-random-session-too
-SHEET_ID=1raMSWRZw_JfHlWqOb3LbhaQ6LWx0VGblxIV4Z2pSzp8
 # Booking confirmation email (#18). Left blank, sending is skipped (logged,
 # not fatal) -- mail relay setup is a separate infra/triage item.
 SMTP_HOST=