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/seed_demo.py

@@ -252,7 +252,7 @@ try:
     for position, s in enumerate(SCRIPTS):
         db.add(models.AdventureScript(adventure_id=adventure.id, position=position, **s))
     opening = models.Action(
-        adventure_id=adventure.id, index=0, type="start", text=scenario.prompt
+        adventure_id=adventure.id, type="start", text=scenario.prompt
     )
     # Through the same door create_adventure uses, so the seeded adventure has a
     # story tree like every other one.