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/config.py

@@ -153,6 +153,19 @@ class Settings(BaseSettings):
         ),
     )
 
+    subscription_overage_percent: int = Field(
+        default=20,
+        ge=0,
+        le=200,
+        description=(
+            "Soft-limit overage buffer in percent (0–200). The announced monthly quota is "
+            "increased by this percentage for actual enforcement. E.g. 20 means a 150-doc/month "
+            "plan enforces at 180 docs (150 × 1.20). Set 0 to enforce exactly at the announced "
+            "limit. Per-plan overage_percent (set in Plan Designer) overrides this global default. "
+            "Default: 20."
+        ),
+    )
+
     # Authentik
     authentik_client_id: Optional[str] = None
     authentik_client_secret: Optional[str] = None