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>

app/main.py

@@ -99,6 +99,19 @@ async def lifespan(app: FastAPI):
     # Send startup notification
     notify_startup()
 
+    # Seed default subscription plans if none exist
+    try:
+        from app.database import SessionLocal as _SessionLocal
+        from app.utils.subscription import seed_default_plans as _seed_plans
+
+        _db_seed = _SessionLocal()
+        try:
+            _seed_plans(_db_seed)
+        finally:
+            _db_seed.close()
+    except Exception:
+        logging.debug("Subscription plan seeding skipped — DB may not be ready yet")  # noqa: S110
+
     # Application is now running
     yield