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/tests/test_mailer.py

@@ -1,7 +1,6 @@
 """Unit tests for mailer.py's SMTP call shape (#18). Calls _send_now directly
 rather than going through the background-thread queue, so assertions are
-synchronous -- the queue itself is just plumbing, already covered indirectly
-by app.py's identical Sheets-mirror pattern.
+synchronous -- the queue itself is just plumbing.
 """
 from email.message import EmailMessage