Phase 10: Per-User Rate Limits — messages + tokens, quota UI, admin usage

Backend:
- max_ai_messages_per_day + max_ai_tokens_per_day on User model (nullable, override)
- Migration 008: add columns + seed default settings (100 msgs, 500K tokens)
- usage_service: count today's messages + tokens, check quota, get limits
- GET /chats/quota returns usage vs limits + reset time
- POST /chats/{id}/messages checks quota before streaming (429 if exceeded)
- Admin user schemas expose both limit fields
- GET /admin/usage returns per-user daily message + token counts
- admin_user_service allows updating both limit fields

Frontend:
- Chat header shows "X/Y messages · XK/YK tokens" with red highlight at limit
- Quota refreshes every 30s via TanStack Query
- Admin usage page with table: user, messages today, tokens today
- Route + sidebar entry for admin usage
- English + Russian translations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

GeneralPlan.md

@@ -258,10 +258,10 @@ Daily scheduled job (APScheduler, 8 AM) reviews each user's memory + recent docs
 - Summary: OAuth (Google, GitHub), account switching UI, multiple stored sessions
 
 ### Phase 10: Per-User Rate Limits
-- **Status**: NOT STARTED
-- [ ] Subplan created (`plans/phase-10-rate-limits.md`)
-- [ ] Phase completed
-- Summary: Per-user AI message rate limits, admin-configurable limits, usage tracking
+- **Status**: COMPLETED
+- [x] Subplan created (`plans/phase-10-rate-limits.md`)
+- [x] Phase completed
+- Summary: Per-user AI message + token rate limits, admin-configurable defaults, usage tracking dashboard
 
 ---