Add booking stack, client deploys, and back-office updates

- deploy/booking: shared Easy!Appointments stack with brand-matched
  wizard (flatpickr recolor, single-tenant provider hide, iframe
  auto-fit height reporter)
- deploy/clients: per-client isolated nginx compose stacks with
  _template scaffold, new-client.sh, and happynails live site
- deploy/backup: smb-db backup script
- n8n: booking-sync workflow; onboarding tweaks
- playbooks: lead-to-customer lifecycle + outreach
- templates: nail-studio landing previews
- backoffice: app/db/init/compose updates

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

deploy/clients/_template/.env.example

@@ -0,0 +1,7 @@
+# One per client. Copy to .env and fill (new-client.sh does this for you).
+CLIENT_SLUG=happynails          # lowercase, no spaces — used for the Compose project + container name
+CLIENT_DOMAIN=happynails.de     # the domain / subdomain Caddy serves for this client
+
+# --- reference only (NOT injected; the site/ HTML is static) ---
+# CLIENT_NAME=Happy Nails Nürnberg
+# BOOKING_URL=https://calendar.app.google/xxxxxxxx   # bake into site/index.html during onboarding