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>

frontend/templates/base.html

@@ -63,6 +63,7 @@
             <a href="/upload" class="text-gray-700 hover:text-gray-900" {% if request and request.url.path == '/upload' %}aria-current="page"{% endif %}>Upload</a>
             <a href="/files" class="text-gray-700 hover:text-gray-900" {% if request and request.url.path == '/files' %}aria-current="page"{% endif %}>Files</a>
             <a href="/search" class="text-gray-700 hover:text-gray-900" {% if request and request.url.path == '/search' %}aria-current="page"{% endif %}>Search</a>
+            <a href="/pricing" class="text-gray-700 hover:text-gray-900" {% if request and request.url.path == '/pricing' %}aria-current="page"{% endif %}>Pricing</a>
 
             <!-- Admin dropdown (shown only for admin users via JS) -->
             <div id="adminMenuContainer" class="relative hidden">
@@ -171,6 +172,7 @@
               <a href="/upload" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Upload</a>
               <a href="/files" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Files</a>
               <a href="/search" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Search</a>
+              <a href="/pricing" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">Pricing</a>
 
               <!-- Admin section in mobile menu (shown only for admin users via JS) -->
               <div id="mobileAdminSection" class="hidden">