- Fix ReferenceError: url_for is not defined in pricing JS (was calling
  server-side Jinja2 helper from client-side JavaScript)
- Add new POST /dashboard/subscribe/{tier_id} endpoint with smart payment
  logic: deducts from wallet if sufficient, otherwise charges saved Stripe
  card for the exact plan amount; returns clear error when neither is
  available
- Add POST /dashboard/subscribe/free for downgrade flow
- Fix plan description field names (max_requests_day → max_requests_per_day,
  etc.) across pricing.html and subscription.html; numbers now formatted
  with thousands separators
- Fix pricing card layout so a single plan is centered and capped in width
  instead of stretching full-screen
- Add is_default and is_active to get_user_tier() return dict
- Add upgrade CTA banner in dashboard overview subscription section when
  higher plans are available
- Add subscription hint banner in wallet page
- Add new Usage & Quotas page (/dashboard/usage) with progress bars for
  daily/monthly requests, providers, rotations, autoselections, and tokens;
  bars warn at 75% and turn red at 90%
- Add Usage link to main nav and account dropdown
- Register usage.html in setup.py data_files