New-client onboarding: provision resources/services/owner login, drop EA (#24)

n8n/onboarding.json now provisions a default resource (with Mon-Sat 09:00-18:00
hours so the public booking page has slots immediately), a starter service, and
an owner-login user for every new client, recording the temp password via the
existing credentials CRM entity -- gated behind an If check so a failed user
creation can't leave a stale credentials row. The EA-provisioning chain
(service/provider creation against Easy!Appointments) is removed entirely.

Adds POST /api/resources, /api/services, /api/owner_users to the backoffice API
for n8n to call, backed by booking_db.py's existing tenancy-safe create_*
helpers. Also adds "slug" to db.py's clients column list -- it was already a DB
column (#17) but the generic /api/clients POST silently dropped it, so
onboarding could never actually set a client's public-facing slug.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backoffice/app/db.py

@@ -20,7 +20,7 @@ TABLES = {
         "cols": ["client_id", "business_name", "owner_name", "email", "phone",
                  "niche", "tier", "status", "domain", "stack_notes", "vault_ref",
                  "services", "billing_cycle", "monthly_fee_eur", "start_date",
-                 "renewal_date", "created_at", "notes", "notify_channel"],
+                 "renewal_date", "created_at", "notes", "notify_channel", "slug"],
         "dates": ["start_date", "renewal_date"],
         "timestamps": ["created_at"],
         "numbers": ["monthly_fee_eur"],