feat(subscriptions): add SaaS subscription tiers, pricing page, and enforced upload quotas

- Add Free / Starter / Professional / Business tiers with lifetime, daily, and monthly
  file limits (app/utils/subscription.py)
- Add subscription_tier column to UserProfile model + migration 014
- Enforce quotas at upload time (HTTP 402 on violation) in /api/ui-upload
- New REST API: GET /api/subscriptions/tiers, /my, /platform (admin)
- New pages: /pricing (marketing, public) and /subscription (per-user status)
- Enhanced dashboard: SaaS stats (files today/month, OCR count, active users)
  in multi-user mode; original single-user layout preserved
- Admin users page: show Plan badge, allow tier editing via dropdown
- Navigation: add Pricing link + subscription icon in user header
- Tests: 23 unit tests for subscription tier logic
- Docs: docs/SubscriptionTiers.md

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/__init__.py

@@ -23,6 +23,7 @@ from app.api.saved_searches import router as saved_searches_router
 from app.api.search import router as search_router
 from app.api.settings import router as settings_router
 from app.api.similarity import router as similarity_router
+from app.api.subscriptions import router as subscriptions_router
 from app.api.url_upload import router as url_upload_router
 
 # Import all the individual routers
@@ -56,3 +57,4 @@ router.include_router(similarity_router)
 router.include_router(duplicates_router)
 router.include_router(webhooks_router)
 router.include_router(database_router)
+router.include_router(subscriptions_router)