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/pricing.html

@@ -170,19 +170,6 @@
               {% endfor %}
             </tr>
 
-            <tr class="hover:bg-indigo-50/30 transition">
-              <td class="px-6 py-3 text-sm text-gray-700">Files per day</td>
-              {% for tier in tiers %}
-              <td class="px-4 py-3 text-center text-sm">
-                {% if tier.daily_upload_limit == 0 %}
-                <span class="font-semibold text-green-600">Unlimited</span>
-                {% else %}
-                <span class="font-medium text-gray-800">{{ tier.daily_upload_limit }}</span>
-                {% endif %}
-              </td>
-              {% endfor %}
-            </tr>
-
             <tr class="hover:bg-indigo-50/30 transition">
               <td class="px-6 py-3 text-sm text-gray-700">Files per month</td>
               {% for tier in tiers %}