feat: consolidate media service, add merchant users page, fix metrics overlap

- Merge ImageService into MediaService with WebP variant generation,
  DB-backed storage stats, and module-driven media usage discovery
  via new MediaUsageProviderProtocol
- Add merchant users admin page with scoped user listing, stats
  endpoint, template, JS, and i18n strings (de/en/fr/lb)
- Fix merchant user metrics so Owners and Team Members are mutually
  exclusive (filter team_members on user_type="member" and exclude
  owner IDs) ensuring stat cards add up correctly
- Update billing and monitoring services to use media_service
- Update subscription-billing and feature-gating docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/implementation/feature-gating-system.md

@@ -354,7 +354,6 @@ The subscription tiers admin page provides full CRUD functionality for managing
    - Code (colored badge by tier)
    - Name
    - Monthly/Annual pricing
-   - Limits (orders, products, team members)
    - Feature count
    - Status (Active/Private/Inactive)
    - Actions (Edit Features, Edit, Activate/Deactivate)
@@ -362,7 +361,6 @@ The subscription tiers admin page provides full CRUD functionality for managing
 3. **Create/Edit Modal**: Form with all tier fields:
    - Code and Name
    - Monthly and Annual pricing (in cents)
-   - Order, Product, and Team member limits
    - Display order
    - Stripe IDs (optional)
    - Description
@@ -371,9 +369,10 @@ The subscription tiers admin page provides full CRUD functionality for managing
 4. **Feature Assignment Slide-over Panel**:
    - Opens when clicking the puzzle-piece icon
    - Shows all features grouped by category
-   - Checkbox selection with Select all/Deselect all per category
+   - Binary features: checkbox selection with Select all/Deselect all per category
+   - Quantitative features: checkbox + numeric limit input for `limit_value`
    - Feature count in footer
-   - Save to update tier's feature assignments
+   - Save to update tier's feature assignments via `TierFeatureLimitEntry[]`
 
 ### Files
 
@@ -392,10 +391,9 @@ The subscription tiers admin page provides full CRUD functionality for managing
 | Create tier | POST | `/api/v1/admin/subscriptions/tiers` |
 | Update tier | PATCH | `/api/v1/admin/subscriptions/tiers/{code}` |
 | Delete tier | DELETE | `/api/v1/admin/subscriptions/tiers/{code}` |
-| Load features | GET | `/api/v1/admin/features` |
-| Load categories | GET | `/api/v1/admin/features/categories` |
-| Get tier features | GET | `/api/v1/admin/features/tiers/{code}/features` |
-| Update tier features | PUT | `/api/v1/admin/features/tiers/{code}/features` |
+| Load feature catalog | GET | `/api/v1/admin/subscriptions/features/catalog` |
+| Get tier feature limits | GET | `/api/v1/admin/subscriptions/features/tiers/{code}/limits` |
+| Update tier feature limits | PUT | `/api/v1/admin/subscriptions/features/tiers/{code}/limits` |
 
 ## Migration