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/docker-compose.yml

@@ -27,16 +27,12 @@ services:
       BOOKING_TOKEN_SECRET: ${BOOKING_TOKEN_SECRET}
       SESSION_SECRET_KEY: ${SESSION_SECRET_KEY}
       ICS_TOKEN: ${ICS_TOKEN}
-      SHEET_ID: ${SHEET_ID}
-      GOOGLE_SA_JSON: /run/secrets/gcp-sa.json
       SMTP_HOST: ${SMTP_HOST}
       SMTP_PORT: ${SMTP_PORT}
       SMTP_USERNAME: ${SMTP_USERNAME}
       SMTP_PASSWORD: ${SMTP_PASSWORD}
       MAIL_FALLBACK_FROM: ${MAIL_FALLBACK_FROM}
       PUBLIC_BASE_URL: ${PUBLIC_BASE_URL}
-    volumes:
-      - ./secrets/gcp-sa.json:/run/secrets/gcp-sa.json:ro
     depends_on:
       smb-db:
         condition: service_healthy