chore: opencode冗余清理 + LLM任务级模型选择 + systemd服务化

- 删除 opencode_search.py / mcp_search_server.py 及所有 MCP 引用
- 移除搜索缓存定时任务（scheduled_refresh_search_cache）
- 清理前后端所有 opencode/MCP 代码和注释
- LLM 提供商量换：opencode-go→nvidia(默认)+sensenova(合规审查)
- llm_configs 新增 is_default 字段，API 层互斥逻辑
- 所有定时任务支持独立 LLM 模型选择（LLM_TASK_PROVIDER env）
- compliance_optimizer.py 修复：import os / 解硬编码 / 关键词过滤
- Scheduler 日志修复：始终 INSERT，避免僵尸 running 行
- Systemd 服务化：Restart=always / 单 worker / Type=exec
- 搜索提供商：替换 opencode→360/搜狗/微信（免 Key）
- 更新 AGENTS.md / PROGRESS.md

platform/backend/app/api/system.py

@@ -280,28 +280,6 @@ def trigger_metrics_sync():
     except Exception as e:
         raise HTTPException(status_code=500, detail=str(e))
 
-@router.post("/refresh-search-cache/run")
-def trigger_refresh_search_cache(db: Session = Depends(get_db), current_user=Depends(get_current_user)):
-    try:
-        import sys as sys_mod
-        scripts_dir = PROJECT_ROOT / "scripts"
-        from ..database import SessionLocal as _ss
-        proc = subprocess.Popen(
-            [sys_mod.executable, str(scripts_dir / "opencode_search.py"), "--refresh-cache"],
-            stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True,
-            cwd=str(PROJECT_ROOT)
-        )
-        logger.info("Search cache refresh started (pid=%s)", proc.pid)
-        log = TaskLog(module_id="scheduled_refresh_search_cache", task_name="🔍 搜索缓存", status="running", message="搜索缓存刷新已启动", triggered_by="manual", started_at=datetime.now(timezone.utc), result_data={"pid": proc.pid})
-        db.add(log)
-        db.commit()
-        log_id = log.id
-        t = threading.Thread(target=_monitor_subprocess, args=(log_id, proc, "scheduled_refresh_search_cache", "🔍 搜索缓存", _ss), daemon=True)
-        t.start()
-        return {"message": "搜索缓存刷新已后台启动", "pid": proc.pid, "log_id": log_id}
-    except Exception as e:
-        raise HTTPException(status_code=500, detail=str(e))
-
 @router.post("/trends/run")
 def trigger_trends_refresh(db: Session = Depends(get_db), current_user=Depends(get_current_user)):
     try:
@@ -370,7 +348,6 @@ def get_modules_status(db: Session = Depends(get_db)):
     config_map = {c.module_id: c for c in configs}
 
     MODULE_META = {
-        "scheduled_refresh_search_cache": {"name": "🔍 搜索缓存", "cron": "01:00", "params_desc": {"refresh_queries": "搜索关键词列表"}},
         "scheduled_fetch_trends": {"name": "🔥 热点趋势", "cron": "01:10", "params_desc": {}},
         "scheduled_collect": {"name": "📡 内容采集", "cron": "01:30", "params_desc": {"max_topics": "最大选题数", "categories": "采集类别"}},
         "scheduled_generate": {"name": "🤖 内容创作", "cron": "02:00", "params_desc": {"auto_review": "自动合规审查"}},