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.",