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/models.py

@@ -181,6 +181,10 @@ class Action(Base):
     # Reasoning-model "thinking" that preceded the text (AI actions only).
     reasoning: Mapped[str | None] = mapped_column(Text, nullable=True)
     context_snapshot: Mapped[dict | None] = mapped_column(JSON, nullable=True)
+    # Copy of Adventure.script_state as it was immediately BEFORE this action's
+    # script hooks ran, so undo/retry can roll the shared scoreboard back.
+    # NULL for actions created before this column existed.
+    state_before: Mapped[dict | None] = mapped_column(JSON, nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime, default=utcnow)
 
     adventure: Mapped[Adventure] = relationship(back_populates="actions")