refactor: restructure services and add image/pdf extraction utilities

- Add image_extractor, pdf_downloader, pi_client, trends services
- Add shared utils module
- Refactor summarizer, embedder, routes for cleaner separation
- Update tests to match new service structure

tests/conftest.py

@@ -15,13 +15,13 @@ from sqlalchemy.pool import StaticPool
 
 from app.database import get_db
 from app.main import create_app
+from app.database import init_db
 from app.models import (
     Paper,
     PaperAuthor,
     PaperSummary,
     PaperTag,
     SummaryStatus,
-    init_db,
 )