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/static/index.html

@@ -74,7 +74,7 @@
     <h1>🗂️ smb-crm · Back Office</h1>
     <div class="hgroup">
       <a class="btn ghost" href="/">📝 Onboarding-Formular</a>
-      <span class="src">Quelle: Postgres (Sheets = Spiegel) · <span id="now"></span></span>
+      <span class="src">Quelle: Postgres · <span id="now"></span></span>
     </div>
   </header>
   <div class="wrap">