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/db.py

@@ -1,8 +1,8 @@
 """Postgres access + the table/column contract shared across the app.
 
 The TABLES map is the single definition of which entities exist, their columns,
-and their primary key. Read/CRUD endpoints, the importer and the Sheets mirror
-all derive from it so they can never drift apart.
+and their primary key. Read/CRUD endpoints all derive from it so they can
+never drift apart.
 """
 import os
 import re
@@ -13,10 +13,9 @@ from psycopg.rows import dict_row
 
 DATABASE_URL = os.environ["DATABASE_URL"]
 
-# entity -> (sheet tab, primary key, ordered columns)
+# entity -> (primary key, ordered columns)
 TABLES = {
     "clients": {
-        "tab": "Clients",
         "pk": "client_id",
         "cols": ["client_id", "business_name", "owner_name", "email", "phone",
                  "niche", "tier", "status", "domain", "stack_notes", "vault_ref",
@@ -28,7 +27,6 @@ TABLES = {
         "bools": [],
     },
     "leads": {
-        "tab": "Leads",
         "pk": "lead_id",
         "cols": ["lead_id", "received_at", "client_id", "source", "name",
                  "contact", "service_interest", "message", "status", "notified"],
@@ -38,7 +36,6 @@ TABLES = {
         "bools": ["notified"],
     },
     "projects": {
-        "tab": "Projects",
         "pk": "project_id",
         "cols": ["project_id", "client_id", "deliverable", "tier", "checklist",
                  "go_live_date", "status"],
@@ -48,7 +45,6 @@ TABLES = {
         "bools": [],
     },
     "bookings": {
-        "tab": "Bookings",
         "pk": "booking_id",
         "cols": ["booking_id", "created_at", "client_id", "customer_name",
                  "customer_contact", "service", "start_time", "end_time",
@@ -59,7 +55,6 @@ TABLES = {
         "bools": [],
     },
     "invoices": {
-        "tab": "Invoices",
         "pk": "invoice_id",
         "cols": ["invoice_id", "client_id", "issued_date", "due_date",
                  "amount_eur", "period", "status", "paid_date"],
@@ -69,7 +64,6 @@ TABLES = {
         "bools": [],
     },
     "activity_log": {
-        "tab": "Activity Log",
         "pk": "id",
         "cols": ["ts", "workflow", "client_id", "action", "detail", "result"],
         "dates": [],
@@ -78,14 +72,13 @@ TABLES = {
         "bools": [],
     },
     "credentials": {
-        # No "tab": never mirrored to Sheets (see "mirror" below) — secrets stay in Postgres only.
+        # Secrets stay in Postgres only.
         "pk": "cred_id",
         "cols": ["cred_id", "client_id", "label", "username", "secret", "notes", "created_at"],
         "dates": [],
         "timestamps": ["created_at"],
         "numbers": [],
         "bools": [],
-        "mirror": False,
     },
 }
 
@@ -94,7 +87,7 @@ def connect():
     return psycopg.connect(DATABASE_URL, row_factory=dict_row)
 
 
-# ---- coercion: Sheet strings / JSON values -> typed Python for Postgres ----
+# ---- coercion: JSON request values -> typed Python for Postgres ----
 
 def parse_date(v):
     if v in (None, ""):