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/db/init.sql

@@ -1,6 +1,4 @@
--- smb-crm — source-of-truth schema (Postgres).
--- Google Sheets is a one-way downstream mirror fed by the DB->Sheets sync.
--- Column sets mirror the Sheet tabs so the mirror stays 1:1.
+-- smb-crm — sole source-of-truth schema (Postgres).
 
 CREATE TABLE IF NOT EXISTS clients (
     client_id        text PRIMARY KEY,
@@ -87,8 +85,8 @@ CREATE TABLE IF NOT EXISTS invoices (
 );
 
 -- Per-client login credentials (e.g. the auto-generated Easy!Appointments
--- provider login). Deliberately NOT mirrored to Sheets — see db.py TABLES
--- ("mirror": False) — so secrets never leave Postgres.
+-- provider login). Gated by X-CRM-Token even to read, so secrets never leave
+-- Postgres.
 CREATE TABLE IF NOT EXISTS credentials (
     cred_id      text PRIMARY KEY,
     client_id    text,