feat(subscriptions): database-backed plan designer with admin CRUD and overage buffer

- Add SubscriptionPlan model and subscription_plans table (migration 015)
- Add billing cycle/period/allow_overage fields to UserProfile (migration 016)
- Add subscription_overage_percent config field (replaces overage_factor)
- Rewrite check_upload_allowed: use overage_percent, yearly carry-over, no daily cap
- Add seed_default_plans(), _plan_to_dict(), get_year_file_count(), _months_elapsed()
- Update get_tier/get_all_tiers to be DB-first with TIER_DEFAULTS fallback
- Add TIER_DEFAULTS alias (TIERS kept for backward compat)
- New /api/plans/ CRUD endpoints (admin-only except list/get)
- New /admin/plans Plan Designer page with Alpine.js UI
- Add Plan Designer link to admin navigation in base.html
- Remove 'Files per day' row from pricing comparison table
- Add billing cycle + period start to admin users edit modal
- Seed default plans on startup in lifespan handler
- Rewrite docs/SubscriptionTiers.md with full plan/overage/API docs
- Fix all tests in test_subscription.py (remove daily cap tests, add overage/carry-over tests)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/templates/base.html

@@ -99,6 +99,9 @@
                   <a href="/admin/users" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
                     <i class="fas fa-users w-4 mr-2 text-blue-500" aria-hidden="true"></i> Users
                   </a>
+                  <a href="/admin/plans" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
+                    <i class="fas fa-layer-group w-4 mr-2 text-indigo-500" aria-hidden="true"></i> Plan Designer
+                  </a>
                   <a href="/admin/credentials" role="menuitem" class="flex items-center px-4 py-2 text-sm text-gray-700 hover:bg-gray-100">
                     <i class="fas fa-key w-4 mr-2 text-yellow-500" aria-hidden="true"></i> Credentials
                   </a>
@@ -186,6 +189,9 @@
                   <a href="/admin/users" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
                     <i class="fas fa-users mr-2 text-blue-400" aria-hidden="true"></i> Users
                   </a>
+                  <a href="/admin/plans" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
+                    <i class="fas fa-layer-group mr-2 text-indigo-400" aria-hidden="true"></i> Plan Designer
+                  </a>
                   <a href="/admin/credentials" class="block px-3 py-3 rounded-md text-base font-medium text-gray-700 hover:text-gray-900 hover:bg-gray-50">
                     <i class="fas fa-key mr-2 text-yellow-400" aria-hidden="true"></i> Credentials
                   </a>