docs: update rate limit values to match config-based system

- Update USAGE_TRACKING.md with new tier limits (50, 200, 1000, unlimited)
- Update AI_INTEGRATION.md with new tier limits
- Add note that limits are loaded from config (ai_calls_per_day)
- Document GET /api/v1/usage endpoint
- Update examples to show is_unlimited field
- Fix test examples with correct limit values

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

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

api/docs/AI_INTEGRATION.md

@@ -473,14 +473,18 @@ monthly = tracker.get_monthly_cost("user_123", 2025, 11)
 
 Tier-based daily limits enforced via `app/services/rate_limiter_service.py`.
 
+Limits are loaded from config (`rate_limiting.tiers.{tier}.ai_calls_per_day`).
+
 ### AI Calls (Turns)
 
 | Tier | Daily Limit |
 |------|------------|
-| FREE | 20 turns |
-| BASIC | 50 turns |
-| PREMIUM | 100 turns |
-| ELITE | 200 turns |
+| FREE | 50 turns |
+| BASIC | 200 turns |
+| PREMIUM | 1000 turns |
+| ELITE | Unlimited |
+
+A value of `-1` in config means unlimited.
 
 ### Custom Actions