fix: prevent InFailedSQLTransactionError in session creation

Root cause: embedding generation could break the DB transaction via a failed
SQL statement. The except block caught the Python error but left the transaction
in a failed state. Subsequent queries (_record_usage → subscription lookup)
then failed with InFailedSQLTransactionError.

Fixes:
- session_embedding_service: use begin_nested() savepoint so failures don't
  poison the parent transaction
- ai_sessions.py: add db.rollback() before _record_usage in all 3 error
  handlers (create, respond, pickup) to recover from broken transactions

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/endpoints/ai_sessions.py

@@ -139,13 +139,18 @@ async def create_session(
         )
     except Exception as e:
         logger.exception("FlowPilot session start failed: %s", e)
-        await _record_usage(
-            current_user, db,
-            generation_type="flowpilot_start",
-            input_tokens=0, output_tokens=0,
-            succeeded=False, error_code=type(e).__name__,
-        )
-        await db.commit()
+        # Rollback the failed transaction before attempting usage recording
+        await db.rollback()
+        try:
+            await _record_usage(
+                current_user, db,
+                generation_type="flowpilot_start",
+                input_tokens=0, output_tokens=0,
+                succeeded=False, error_code=type(e).__name__,
+            )
+            await db.commit()
+        except Exception:
+            logger.warning("Failed to record usage after session start failure", exc_info=True)
         raise HTTPException(
             status_code=status.HTTP_502_BAD_GATEWAY,
             detail=f"AI provider error ({type(e).__name__}). Please try again.",
@@ -193,15 +198,19 @@ async def respond_to_step(
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=str(e))
     except Exception as e:
         logger.exception("FlowPilot response failed: %s", e)
-        await _record_usage(
-            current_user, db,
-            generation_type="flowpilot_respond",
-            input_tokens=0, output_tokens=0,
-            succeeded=False,
-            session_id=session_id,
-            error_code=type(e).__name__,
-        )
-        await db.commit()
+        await db.rollback()
+        try:
+            await _record_usage(
+                current_user, db,
+                generation_type="flowpilot_respond",
+                input_tokens=0, output_tokens=0,
+                succeeded=False,
+                session_id=session_id,
+                error_code=type(e).__name__,
+            )
+            await db.commit()
+        except Exception:
+            logger.warning("Failed to record usage after response failure", exc_info=True)
         raise HTTPException(
             status_code=status.HTTP_502_BAD_GATEWAY,
             detail=f"AI provider error ({type(e).__name__}). Please try again.",
@@ -387,15 +396,19 @@ async def pickup_session(
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail=str(e))
     except Exception as e:
         logger.exception("FlowPilot pickup failed: %s", e)
-        await _record_usage(
-            current_user, db,
-            generation_type="flowpilot_pickup",
-            input_tokens=0, output_tokens=0,
-            succeeded=False,
-            session_id=session_id,
-            error_code=type(e).__name__,
-        )
-        await db.commit()
+        await db.rollback()
+        try:
+            await _record_usage(
+                current_user, db,
+                generation_type="flowpilot_pickup",
+                input_tokens=0, output_tokens=0,
+                succeeded=False,
+                session_id=session_id,
+                error_code=type(e).__name__,
+            )
+            await db.commit()
+        except Exception:
+            logger.warning("Failed to record usage after pickup failure", exc_info=True)
         raise HTTPException(
             status_code=status.HTTP_502_BAD_GATEWAY,
             detail=f"AI provider error ({type(e).__name__}). Please try again.",

backend/app/services/session_embedding_service.py

@@ -54,44 +54,44 @@ async def generate_session_embedding(session_id: UUID, db: AsyncSession) -> None
 
         embedding_str = "[" + ",".join(str(v) for v in embedding_vector) + "]"
 
-        # Check for existing embedding
-        existing = await db.execute(
-            select(AISessionEmbedding).where(
-                AISessionEmbedding.session_id == session_id
+        # Use a savepoint so failures don't poison the parent transaction
+        async with db.begin_nested():
+            # Check for existing embedding
+            existing = await db.execute(
+                select(AISessionEmbedding).where(
+                    AISessionEmbedding.session_id == session_id
+                )
             )
-        )
-        embed_record = existing.scalar_one_or_none()
+            embed_record = existing.scalar_one_or_none()
 
-        if embed_record:
-            # Update existing
-            embed_record.chunk_text = chunk_text
-            await db.execute(
-                text(
-                    "UPDATE ai_session_embeddings "
-                    "SET embedding = :emb::vector, updated_at = now() "
-                    "WHERE session_id = :sid"
-                ),
-                {"emb": embedding_str, "sid": str(session_id)},
-            )
-        else:
-            # Insert new via raw SQL to include vector column
-            await db.execute(
-                text("""
-                    INSERT INTO ai_session_embeddings
-                        (id, session_id, account_id, chunk_text, embedding_model, embedding, created_at, updated_at)
-                    VALUES
-                        (gen_random_uuid(), :session_id, :account_id, :chunk_text, :model, :embedding::vector, now(), now())
-                """),
-                {
-                    "session_id": str(session_id),
-                    "account_id": str(session.account_id),
-                    "chunk_text": chunk_text,
-                    "model": "voyage-3.5",
-                    "embedding": embedding_str,
-                },
-            )
-
-        await db.flush()
+            if embed_record:
+                # Update existing
+                embed_record.chunk_text = chunk_text
+                await db.execute(
+                    text(
+                        "UPDATE ai_session_embeddings "
+                        "SET embedding = :emb::vector, updated_at = now() "
+                        "WHERE session_id = :sid"
+                    ),
+                    {"emb": embedding_str, "sid": str(session_id)},
+                )
+            else:
+                # Insert new via raw SQL to include vector column
+                await db.execute(
+                    text("""
+                        INSERT INTO ai_session_embeddings
+                            (id, session_id, account_id, chunk_text, embedding_model, embedding, created_at, updated_at)
+                        VALUES
+                            (gen_random_uuid(), :session_id, :account_id, :chunk_text, :model, :embedding::vector, now(), now())
+                    """),
+                    {
+                        "session_id": str(session_id),
+                        "account_id": str(session.account_id),
+                        "chunk_text": chunk_text,
+                        "model": "voyage-3.5",
+                        "embedding": embedding_str,
+                    },
+                )
     except Exception:
         logger.warning(
             "Failed to generate embedding for session %s", session_id, exc_info=True