fix: session handling and SDK async iteration

- Add sdk_conversation_id column to track SDK conversation separately from user session_id
- Queue processor looks up SDK conversation ID for resume instead of passing user session_id
- session_id "new" stores NULL initially, gets set from SDK on completion
- Custom session_ids are preserved (not overwritten by SDK)
- Worker catches SDK process exit error after messages are collected

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/backend/services/queue.py

@@ -9,6 +9,7 @@ from src.backend.db.database import (
     complete_request,
     fail_request,
     append_topic_summary,
+    get_session_sdk_id,
 )
 from src.backend.services.worker_client import WorkerClient
 
@@ -38,6 +39,11 @@ async def process_one(worker: WorkerClient) -> bool:
             return True
 
         model = resolve_model(req["model"], topic["model"])
+
+        # Only resume if there's a previous completed request with an SDK conversation ID
+        resume_id = None
+        if req["session_id"]:
+            resume_id = await get_session_sdk_id(db, req["session_id"])
     finally:
         await db.close()
 
@@ -46,7 +52,7 @@ async def process_one(worker: WorkerClient) -> bool:
             prompt=req["prompt"],
             system_prompt=topic["system_prompt"],
             model=model,
-            session_id=req["session_id"],
+            session_id=resume_id,
         )
     except Exception as e:
         db = await get_db()
@@ -63,7 +69,7 @@ async def process_one(worker: WorkerClient) -> bool:
                 db,
                 req["id"],
                 result["result"],
-                session_id=result.get("session_id"),
+                sdk_conversation_id=result.get("session_id"),
             )
 
             if req["summarize"]: