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

AGENTS.md

@@ -4,9 +4,9 @@
 - **Backend**: FastAPI 0.104 + SQLAlchemy 2.0 + PostgreSQL 16 (`yzr_nr`)
 - **Frontend**: Vue 3 (CDN, no build step) + Element Plus — static HTML served by FastAPI
 - **Auth**: JWT (`python-jose` + bcrypt), default admin `admin/admin123`
-- **Scheduler**: APScheduler (daily cron: 01:00 searchcache, 01:10 trends, 01:30 collect, 02:00 generate, 03:00 optimize, 05:00 sources, 06:00 metrics)
+- **Scheduler**: APScheduler (daily cron: 01:10 trends, 01:30 collect, 02:00 generate, 03:00 optimize, 05:00 sources, 06:00 metrics)
 - **Task DB**: `TaskLog` (module_id/status/error_trace/result_data/triggered_by) + `TaskConfig` (params/enabled/schedule)
-- **LLM**: Multi-provider (opencode-go primary, nvidia backup). API keys only in `.env`, not DB.
+- **LLM**: Multi-provider (nvidia primary, opencode-go fallback). API keys in DB (managed via admin UI) or `.env`.
 
 ## Commands