Drop the eight legacy columns SP8 left behind

Migrations 66 to 73 drop `actions.index`, `variants`, `variant_index`,
`variant_count`, `state_before`, and `world_state_before`, plus
`adventures.memory_cursor` and `summary_cursor`. `index` is a keyword in
SQLite, so migration 71 quotes it.

Nothing outside the migrations read these. `models.py`, `schemas.py`, and
`ACTION_LIST_COLUMNS` lose the same eight fields, `Adventure.actions` orders
by `id`, and `attempts.renumber`, `context.history.max_action_index`, and
`nodes.next_index` are deleted.

Two changes keep the migration replayable on a `create_all` database:

- `_split_variants_into_siblings` wrote through the live ORM table, so it
  stopped compiling once migration 66 removed five of its columns. It now
  writes through `_ACTIONS_AT_60`, a frozen `Table` with its own `MetaData`.
- Five data passes read columns these migrations drop. Each now calls
  `_has_columns` and returns early when the columns are absent.

`bootstrap` takes a `through` version so a migration test can stop at the
schema it asserts on.

555 tests pass, up from 549. Eight of the new cases assert each column is
gone after a real schema-45 database migrates all the way.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0198qDK3gmgSo7EtQ4GTPqqK

backend/tests/test_prompt_caching.py

@@ -134,7 +134,7 @@ def story():
     db.add(adventure)
     db.flush()
     for i in range(6):
-        db.add(models.Action(adventure_id=adventure.id, index=i,
+        db.add(models.Action(adventure_id=adventure.id,
                              type="ai" if i % 2 else "do",
                              text=f"[{i}] The road bends onward past the treeline."))
     db.commit()
@@ -202,7 +202,7 @@ def test_live_sections_are_still_charged_to_the_budget(story):
     db, adventure, settings = story
     for i in range(6, 90):
         db.add(models.Action(
-            adventure_id=adventure.id, index=i, type="do",
+            adventure_id=adventure.id, type="do",
             text=f"[{i}] " + "The road bends onward past the treeline. " * 6,
         ))
     settings.context_token_budget = 4000
@@ -226,7 +226,7 @@ def test_a_new_turn_only_appends_to_the_cached_prefix(story):
     prefix has to still contain the whole static block and the older history."""
     db, adventure, settings = story
     system_a, story_a, _ = builder.build_context(adventure, settings)
-    db.add(models.Action(adventure_id=adventure.id, index=6, type="do",
+    db.add(models.Action(adventure_id=adventure.id, type="do",
                          text="[6] You step into the clearing."))
     db.commit()
     db.expire_all()