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/views/subscriptions.py

@@ -18,9 +18,9 @@ router = APIRouter()
 
 
 @router.get("/pricing", include_in_schema=False)
-async def pricing_page(request: Request):
+async def pricing_page(request: Request, db: Session = Depends(get_db)):
     """Public-facing pricing and plans page."""
-    tiers = get_all_tiers()
+    tiers = get_all_tiers(db)
     return templates.TemplateResponse(
         "pricing.html",
         {
@@ -47,8 +47,8 @@ async def my_subscription_page(request: Request, db: Session = Depends(get_db)):
         tier_id = "business"
         usage = None
 
-    tier = get_tier(tier_id)
-    all_tiers = get_all_tiers()
+    tier = get_tier(tier_id, db)
+    all_tiers = get_all_tiers(db)
 
     return templates.TemplateResponse(
         "subscription.html",