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>

README.md

@@ -2,9 +2,8 @@
 
 A productized service that gets local small businesses online: **landing page + online
 booking + lead capture + small automations**. Built to be **thoroughly tracked and logged** —
-every client, lead, booking and invoice lives in a **Postgres CRM** (source of truth), driven by
-**n8n** on the homelab, with a one-way mirror into Google Sheets for a human-readable overview /
-Looker Studio dashboard.
+every client, lead, booking and invoice lives in a **Postgres CRM** (sole source of truth),
+driven by **n8n** on the homelab.
 
 > Full documentation (architecture, data model, every component): **`Documentation.md`**.
 
@@ -29,7 +28,6 @@ deploy/
   backup/                      Daily Postgres backup script (cron on the homelab)
 n8n/                         Exported n8n workflows (lead-intake, onboarding, booking-sync, renewal-reminder)
 playbooks/                   Operator playbooks (outreach, lead-to-customer, tier-a, tier-b)
-sheets/SCHEMA.md             Google Sheets mirror spec (tabs + columns) — downstream of Postgres
 templates/landing/           Landing-page demos + client preview pages
   demo-tier-a/                  "Schnittpunkt" barbershop demo
   demo-tier-b/                  "PhysioVital" physio practice demo
@@ -65,8 +63,7 @@ Tracked against actual repo state — see `Documentation.md` for what backs each
 - [x] **Phase 1** — Demo landing pages
 - [x] **Phase 2** — Accounts & setup: Google account + workbook, n8n live, Google creds bound,
       Vaultwarden (`clients.vault_ref`), domain (wildcard `*.mivanchenko.de`)
-- [x] **Phase 3** — CRM (now Postgres-first, Sheets mirrored) + n8n lead-intake & renewal
-      workflows — Looker Studio dashboard itself lives in Google and isn't repo-verifiable
+- [x] **Phase 3** — CRM (Postgres-only, no Sheets mirror) + n8n lead-intake & renewal workflows
 - [x] **Phase 4** — Demo forms + booking wired live to n8n webhooks (lead-intake, booking-sync)
 - [ ] **Phase 5** — First real client + reception v1 — onboarding automation and per-client
       deploy tooling exist and are exercised (`deploy/clients/`); not verifiable from the repo