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/app/schemas.py

@@ -186,27 +186,20 @@ class RefreshPlan(BaseModel):
 class ActionOut(ORMModel):
     id: int
     adventure_id: int
-    index: int
     type: str
     text: str
     reasoning: str | None = None
     # Phase 12: the compact RPG state changes for this turn, read from the
     # model property.
     world_changes: list[dict] = []
-    # Retry history: how many attempts exist for this turn, where 0 means the
-    # turn was never retried, and which attempt is live. The attempts themselves
-    # come from `GET /actions/{id}/variants`, so this payload stays small.
-    variant_count: int = 0
-    variant_index: int = 0
     # SP9: the pager, such as `2/4`. It reports how many attempts this turn has
     # and which one is on screen. It is keyed on the parent, so it counts the
     # attempts of this turn rather than every node that shares a depth, and it
     # keeps counting them after one has been forked onto its own branch.
     #
     # A turn nobody has retaken reads 1/1, which is most turns, and the client
-    # draws no pager for a count of one. `variant_count` uses a different
-    # convention and reports 0 for the same case. Those two fields are the
-    # pre-SP9 pair, and SP8 drops them.
+    # draws no pager for a count of one. The attempts themselves come from
+    # `GET /actions/{id}/variants`, so this payload stays small.
     take_count: int = 1
     take_index: int = 0
     # Which line this node is on, so the pager can distinguish the two kinds of