Sweep the last three uses of the columns SP8 dropped

`seed_demo.py` still passed `index=0` to `models.Action`, which raises
`TypeError` now that the attribute is gone. Every call site inside `app/`
was updated when the column went, but the seed script sits outside the
package and was missed.

`bundle.settle` wrote `memory_cursor` and `summary_cursor`, which are no
longer mapped, so the assignments only set transient Python attributes.
The version 2 branch existed solely to make those assignments, and it ran
two queries per cursor to do it, so it goes. `settle` now returns early
when the bundle carries anchors. The `db` parameter is unused after that.

Also move the comment about deleted memories next to the
`db.delete(branch)` it describes. Splitting the router package left it
after the `finally`, where it read as attached to nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YFQY6WgaE3JaX3dXkxLynV

backend/app/routers/adventures/branches.py

@@ -190,18 +190,15 @@ def delete_branch(
             stored_branch, _ = cursor.stored(adventure)
             if stored_branch in doomed:
                 cursor.clear(adventure)
+        # The deleted branch's memories are deleted with it, and their cached
+        # vectors drop out of the catalogue on the next read, so no
+        # invalidation call is needed. See the note on the `memorybank` cache.
         db.delete(branch)
         adventure.updated_at = models.utcnow()
         db.commit()
     finally:
         turns._active_turns.discard(adventure_id)
 
-
-    # The deleted branch's memories are deleted with it, and their cached
-    # vectors drop out of the catalogue on the next read, so no invalidation
-    # call is needed. See the note on the `memorybank` cache.
-
-
 def _branch_subtree(
     db: Session, adventure: models.Adventure, root: models.Branch
 ) -> set[int]: