Show per-turn stat changes under each AI message

Each AI action now carries a compact world_changes summary (derived from its
stored snapshot), rendered as small chips beneath the message: numeric stats
show a signed delta (green up / red down), flags show on/off, milestones show
a check. Gives an at-a-glance "what changed" without opening Insights.

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

backend/app/schemas.py

@@ -131,6 +131,8 @@ class ActionOut(ORMModel):
     type: str
     text: str
     reasoning: str | None = None
+    # Phase 12: compact RPG state changes for this turn (from the model property).
+    world_changes: list[dict] = []
     created_at: datetime