fix(db,frontend): reserve DB connections for API and reduce frontend polling

Background tasks (ingestion, PDF resolution, scheduler) now acquire an
asyncio.Semaphore before checking out a DB connection, guaranteeing at
least N connections remain available for API request handlers. Removes
duplicate polling loop from PublicationsPage, debounces publication
reload on run-status changes, and increases poll interval from 5s to 15s
since SSE handles live discovery.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/api/routers/runs.py

@@ -221,11 +221,11 @@ def _spawn_background_execution(
     user_settings: Any,
     idempotency_key: str | None,
 ) -> None:
-    from app.db.session import get_session_factory
+    from app.db.background_session import background_session
 
     task = asyncio.create_task(
         ingest_service.execute_run(
-            session_factory=get_session_factory(),
+            session_factory=background_session,
             run_id=run.id,
             user_id=current_user.id,
             scholars=scholars,