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

@@ -139,6 +139,9 @@ async def create_session(
         )
     except Exception as e:
         logger.exception("FlowPilot session start failed: %s", e)
+        # Rollback the failed transaction before attempting usage recording
+        await db.rollback()
+        try:
             await _record_usage(
                 current_user, db,
                 generation_type="flowpilot_start",
@@ -146,6 +149,8 @@ async def create_session(
                 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.",
@@ -193,6 +198,8 @@ 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 db.rollback()
+        try:
             await _record_usage(
                 current_user, db,
                 generation_type="flowpilot_respond",
@@ -202,6 +209,8 @@ async def respond_to_step(
                 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.",
@@ -387,6 +396,8 @@ 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 db.rollback()
+        try:
             await _record_usage(
                 current_user, db,
                 generation_type="flowpilot_pickup",
@@ -396,6 +407,8 @@ async def pickup_session(
                 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

@@ -54,6 +54,8 @@ async def generate_session_embedding(session_id: UUID, db: AsyncSession) -> None
 
         embedding_str = "[" + ",".join(str(v) for v in embedding_vector) + "]"
 
+        # 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(
@@ -90,8 +92,6 @@ async def generate_session_embedding(session_id: UUID, db: AsyncSession) -> None
                         "embedding": embedding_str,
                     },
                 )
-
-        await db.flush()
     except Exception:
         logger.warning(
             "Failed to generate embedding for session %s", session_id, exc_info=True