feat(api): Add human feedback API (#6) + async_utils module

Phase 6.6 人類回饋 API:
- PUT /api/v1/incidents/{id}/feedback endpoint
- effectiveness_score (1-5), human_feedback, learning_notes fields
- Sync to Redis (Working Memory) + PostgreSQL (Episodic Memory)
- For stats aggregation at /api/v1/stats/feedback/summary

async_utils module:
- fire_and_forget() for safe background tasks
- Prevents swallowed exceptions in asyncio.create_task()
- Addresses P2 #8 tech debt

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

apps/api/src/core/async_utils.py

@@ -0,0 +1,54 @@
+"""
+Async Utilities
+===============
+Safe async patterns for production use.
+
+ADR-013: 此模組包含關鍵異步模式，修改前請先讀懂註解！
+"""
+
+import asyncio
+from collections.abc import Coroutine
+from typing import Any
+
+from src.core.logging import get_logger
+
+logger = get_logger("awoooi.async_utils")
+
+
+def fire_and_forget(coro: Coroutine[Any, Any, Any], name: str | None = None) -> asyncio.Task[Any]:
+    """
+    安全的 fire-and-forget 模式。
+
+    🔴 重要：不使用此函數的 asyncio.create_task() 會吞掉例外！
+
+    Args:
+        coro: 要執行的協程
+        name: 任務名稱 (用於日誌)
+
+    Returns:
+        已建立的任務 (可忽略)
+
+    Example:
+        # ✅ 正確用法
+        fire_and_forget(send_notification(user_id), name="notification")
+
+        # ❌ 錯誤用法 (例外會被吞掉)
+        asyncio.create_task(send_notification(user_id))
+    """
+    task = asyncio.create_task(coro, name=name)
+
+    def _handle_exception(t: asyncio.Task[Any]) -> None:
+        try:
+            exc = t.exception()
+            if exc is not None:
+                logger.error(
+                    "fire_and_forget_exception",
+                    task_name=name or "unnamed",
+                    error=str(exc),
+                    error_type=type(exc).__name__,
+                )
+        except asyncio.CancelledError:
+            logger.debug("fire_and_forget_cancelled", task_name=name or "unnamed")
+
+    task.add_done_callback(_handle_exception)
+    return task