refactor: extract admin business logic to services, introduce job queue, add derived index helpers

- Move DB operations from routes/admin.py to services/admin.py (get_logs_context, query_summary_statuses, retry_failed, delete/reset operations)
- Add services/jobs.py with Job/JobEvent-based async job queue (create_job, run_job, enqueue_job)
- Add services/derived.py with FTS5 reindex and paper index deletion helpers
- Refactor scheduler to use job queue instead of direct pipeline calls
- Add heartbeat_at/expires_at to TaskLock for lock health tracking
- Remove DESIGN_REVIEW.md
- Update tests: remove redundant integration tests, add unit tests for new services

.env.example

@@ -42,7 +42,7 @@ DATABASE_URL=sqlite:///data/db/papers.db
 # ─── 语义搜索 ─────────────────────────────
 CHROMA_ENABLED=false
 CHROMA_DIR=data/chroma
-EMBED_API_BASE=https://api.siliconflow.cn/v1/embeddings
+EMBED_API_BASE=https://api.siliconflow.cn
 EMBED_API_KEY=your_api_key_here
 EMBED_MODEL=Qwen/Qwen3-Embedding-4B
 EMBED_DIMENSIONS=2560