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/mailer.py

@@ -1,8 +1,8 @@
 """Best-effort SMTP email sending (#18).
 
-Mirrors app.py's DB -> Sheets mirror pattern: a single background worker
-thread drains a queue, and a send failure is logged, never raised back to the
-caller -- booking creation must succeed even if the mail relay is down.
+A single background worker thread drains a queue, and a send failure is
+logged, never raised back to the caller -- booking creation must succeed even
+if the mail relay is down.
 """
 import os
 import queue