Roll back script_state on undo/retry; fix retry double-apply

The shared per-adventure script_state ("scoreboard" scripts write to) was
never reverted by undo, and retry re-ran the output hook on top of the already-
mutated state, double-applying its changes (e.g. "+10 gold" became +20).

Each action now snapshots script_state as it was immediately before its own
hooks ran (new Action.state_before column, migration 25):
- undo restores the turn's first-action snapshot, prunes memories that
  summarized the removed actions, and takes the turn lock against races.
- retry restores the AI action's snapshot before regenerating.

Story-card mutations are not reverted (documented limit). Adds the project's
first test suite: unit + full HTTP integration through the real scripting
engine (14 tests). See plan/11-state-revert-and-retry-fix.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/migrations.py

@@ -75,6 +75,10 @@ MIGRATIONS: list[tuple[int, str]] = [
     # re-synced on demand. NULL for copies made before this column existed.
     (24, "ALTER TABLE adventure_scripts ADD COLUMN source_script_id INTEGER "
          "REFERENCES scripts(id) ON DELETE SET NULL"),
+    # Per-action snapshot of the shared script_state as it was before that
+    # action's hooks ran, enabling undo/retry to roll state back. JSON is valid
+    # on both SQLite and Postgres.
+    (25, "ALTER TABLE actions ADD COLUMN state_before JSON"),
 ]
 
 LATEST_VERSION = max((v for v, _ in MIGRATIONS), default=1)