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/features/subscription-billing.md

@@ -137,6 +137,26 @@ tier.features = [
 ]
 ```
 
+### Admin Tier Management
+
+Administrators can manage subscription tiers at `/admin/subscription-tiers`:
+
+**Capabilities:**
+- View all tiers with stats (total, active, public, MRR)
+- Create new tiers with custom pricing and limits
+- Edit tier properties (name, pricing, limits, Stripe IDs)
+- Activate/deactivate tiers
+- Assign features to tiers via slide-over panel
+
+**Feature Assignment:**
+1. Click the puzzle-piece icon on any tier row
+2. Features are displayed grouped by category
+3. Use checkboxes to select/deselect features
+4. Use "Select all" / "Deselect all" per category
+5. Click "Save Features" to update
+
+See [Feature Gating System](../implementation/feature-gating-system.md#admin-tier-management-ui) for technical details.
+
 ## Limit Enforcement
 
 Limits are enforced at the service layer: