psycopg.errors.ForeignKeyViolation: update or delete on table "trace" violates foreign key constraint "span_trace_id_fkey" on table "span"
DETAIL:  Key (id)=(...) is still referenced from table "span".

And then:

RuntimeError: ('Unable to cascade delete flow: ...', IntegrityError(...))

This was reproduced on langflowai/langflow:1.9.0. I had also seen the same behavior previously on 1.8.x.

Environment:

- Langflow 1.9.0
- PostgreSQL
- tracing data exists for the flow
- deleting flow from the Langflow UI

Additional context:

- This does not appear to be caused by stale local data or a one-off corrupted database.
- The issue is reproducible on a fresh PostgreSQL-backed Langflow setup after creating tracing data and then deleting the flow.
- In my case, fixing the DB constraint manually by changing span.trace_id -> trace.id to ON DELETE CASCADE makes flow deletion work again immediately.

### Reproduction

1. Start Langflow with PostgreSQL as the main database.
2. Create or import a flow.
3. Run the flow at least once so tracing data is written.
4. In the UI, delete that flow.

Result:
- deletion fails
- Langflow logs show `span_trace_id_fkey` blocking the delete

Optional DB verification:

```sql
SELECT conname, pg_get_constraintdef(c.oid) AS definition
FROM pg_constraint c
JOIN pg_class t ON c.conrelid = t.oid
WHERE t.relname = 'span' AND conname = 'span_trace_id_fkey';

Observed result:

FOREIGN KEY (trace_id) REFERENCES trace(id)

So if trace rows are deleted, span rows are left behind unless they are deleted manually first.

### Expected behavior

Deleting a flow should also delete its related tracing data cleanly when PostgreSQL is used.

If a flow is deleted:
- related `trace` rows should be deleted
- related `span` rows should also be deleted automatically
- the UI delete action should succeed without manual database repair

At minimum, the PostgreSQL schema should define `span.trace_id -> trace.id` with `ON DELETE CASCADE`, or Langflow should explicitly delete dependent `span` rows before deleting `trace`/`flow`.

### Who can help?

_No response_

### Operating System

macOS Tahoe 26.4.1 with OrbStack Docker

### Langflow Version

1.9.0

### Python Version

3.12

### Screenshot

_No response_

### Flow File

_No response_