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/api/__init__.py

@@ -17,6 +17,7 @@ from app.api.google_drive import router as google_drive_router
 from app.api.logs import router as logs_router
 from app.api.onedrive import router as onedrive_router
 from app.api.openai import router as openai_router
+from app.api.plans import router as plans_router
 from app.api.process import router as process_router
 from app.api.queue import router as queue_router
 from app.api.saved_searches import router as saved_searches_router
@@ -58,3 +59,4 @@ router.include_router(duplicates_router)
 router.include_router(webhooks_router)
 router.include_router(database_router)
 router.include_router(subscriptions_router)
+router.include_router(plans_router)