feat(ai): $5 USD 成本上限 + 自動切換 Ollama (2026-03-29 ogt)

統帥要求:
1. 累積成本超過 $5 USD → 自動停用 Gemini，切換回 Ollama
2. 發送 Telegram 告警通知統帥
3. $4 USD 時發送警告

實作:
- ai_rate_limiter.py: 新增 COST_LIMITS, record_cost(), reset_cost()
- openclaw.py: 每次成功呼叫後記錄成本
- 成本存入 Redis (不過期，手動重置)
- 重置指令: redis-cli DEL ai_rate:total_cost:gemini

API 端點: GET /api/v1/health/ai-usage
- 顯示 total_cost_usd.current/limit/remaining
- 顯示 cost_exceeded: true/false

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

apps/api/src/services/openclaw.py

@@ -908,6 +908,11 @@ class OpenClawService:
                     )
                     # Langfuse: 記錄成功評分
                     trace.score(name="provider_success", value=1.0, comment=f"Success via {provider}")
+
+                    # 2026-03-29 ogt: 記錄累積成本 (Gemini/Claude)
+                    if cost_usd > 0:
+                        await rate_limiter.record_cost(provider, cost_usd)
+
                     return response, provider, True, total_tokens, cost_usd
 
                 logger.warning("ai_provider_failed_fallback", provider=provider, latency_ms=latency_ms)