feat(subscriptions): migrate subscription management to merchant level and seed tiers

Move subscription create/edit from store detail (broken endpoint) to merchant
detail page with proper modal UI. Seed 4 subscription tiers (Essential,
Professional, Business, Enterprise) in init_production.py. Also includes
cross-module dependency declarations, store domain platform_id migration,
platform context middleware, CMS route fixes, and migration backups.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mkdocs.yml

@@ -221,6 +221,8 @@ nav:
       - Store Onboarding: features/store-onboarding.md
       - Subscription & Billing: features/subscription-billing.md
       - Email System: features/email-system.md
+      - User Journeys:
+        - Loyalty: features/user-journeys/loyalty.md
 
     # --- User Guides ---
     - User Guides: