feat: add feature assignment to admin tier management UI

- Add slide-over panel for assigning features to subscription tiers
- Features grouped by category with select all/deselect all
- Add puzzle-piece icon button in tier table actions
- Add feature management methods to subscription-tiers.js
- Fix JS-006 by adding try/catch to init function

Documentation:
- Update feature-gating-system.md with Admin Tier Management UI section
- Update subscription-billing.md with tier management overview
- Add new admin user guide: subscription-tier-management.md
- Add guide to mkdocs.yml navigation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/implementation/feature-gating-system.md

@@ -338,6 +338,65 @@ Located at `/admin/features`:
 - Search by name/code
 - View tier requirements
 
+## Admin Tier Management UI
+
+Located at `/admin/subscription-tiers`:
+
+### Overview
+
+The subscription tiers admin page provides full CRUD functionality for managing subscription tiers and their feature assignments.
+
+### Features
+
+1. **Stats Cards**: Display total tiers, active tiers, public tiers, and estimated MRR
+2. **Tier Table**: Sortable list of all tiers with:
+   - Display order
+   - 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)
+
+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
+   - Active/Public toggles
+
+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
+   - Feature count in footer
+   - Save to update tier's feature assignments
+
+### Files
+
+| File | Purpose |
+|------|---------|
+| `app/templates/admin/subscription-tiers.html` | Page template |
+| `static/admin/js/subscription-tiers.js` | Alpine.js component |
+| `app/routes/admin_pages.py` | Route registration |
+
+### API Endpoints Used
+
+| Action | Method | Endpoint |
+|--------|--------|----------|
+| Load tiers | GET | `/api/v1/admin/subscriptions/tiers` |
+| Load stats | GET | `/api/v1/admin/subscriptions/stats` |
+| 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` |
+
 ## Migration
 
 The features are seeded via Alembic migration: