fix: 后台思考使用深度思考模型 + 主动消息推送冷却优化

- thinker.go: Round 0 优先调用 llmAdapter(deepseek-v4-pro)，失败回退 toolAdapter
- thinker.go: RecordUserMessage 重置 lastProactiveMsgTime，活跃对话中允许推送
- proactive_decision.go: MinGap low 30→15min, medium 10→5min, high 2→1min; 小时上限 3→5

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/ai-core/internal/background/proactive_decision.go

@@ -36,11 +36,11 @@ func DefaultProactiveGuard() *ProactiveGuard {
 		QuietHoursStart: 23,
 		QuietHoursEnd:   7,
 		MinGapByUrgency: map[string]time.Duration{
-			"low":    30 * time.Minute,
-			"medium": 10 * time.Minute,
-			"high":   2 * time.Minute,
+			"low":    15 * time.Minute,
+			"medium": 5 * time.Minute,
+			"high":   1 * time.Minute,
 		},
-		MaxMessagesPerHour: 3,
+		MaxMessagesPerHour: 5,
 	}
 }