feat: AI 周报摘要提示词自定义设置

- system_config 新增 ai_summary_prompt 配置键
- generate_summary 从数据库加载自定义提示词，为空则使用默认 SUMMARY_SYSTEM_PROMPT
- 系统设置新增「AI 周报提示词」卡片，支持编辑和清空恢复默认

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

backend/app/api/system_config.py

@@ -66,7 +66,7 @@ async def update_config(
 ):
     """Update a single config key. Only directors can change settings."""
     value = body.get("value", "")
-    valid_keys = {"notification_time", "holidays"}
+    valid_keys = {"notification_time", "holidays", "ai_summary_prompt"}
 
     if key not in valid_keys:
         raise HTTPException(status_code=400, detail=f"不支持的配置项: {key}")

backend/app/services/ai_summary.py

@@ -6,6 +6,7 @@ from app.config import settings
 from app.services.dashboard import get_weekly_report, get_week_range
 from app.services.light_board import get_light_board
 from app.models.ai_summary import AISummary
+from app.models.system_config import SystemConfig
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 from uuid import UUID
@@ -240,6 +241,12 @@ async def generate_summary(
         leaves=leave_overview.get("leave_list", []),
     )
 
+    # Load custom system prompt (director/leader can override via system settings)
+    custom_row = await db.get(SystemConfig, "ai_summary_prompt")
+    system_prompt = SUMMARY_SYSTEM_PROMPT
+    if custom_row and custom_row.value and custom_row.value.strip():
+        system_prompt = custom_row.value.strip()
+
     # Call LLM
     headers = {"Content-Type": "application/json"}
     if settings.AI_API_KEY:
@@ -248,7 +255,7 @@ async def generate_summary(
     payload = {
         "model": settings.AI_MODEL,
         "messages": [
-            {"role": "system", "content": SUMMARY_SYSTEM_PROMPT},
+            {"role": "system", "content": system_prompt},
             {"role": "user", "content": user_prompt},
         ],
         "max_tokens": settings.AI_MAX_TOKENS,