Add per-adventure "Sync from library" for scripts

Adventure scripts are frozen snapshots taken at creation. Add an opt-in
way to pull the latest library code into a live adventure:

- AdventureScript.source_script_id links a copy to its library Script
  (migration 24; set at adventure creation)
- list endpoint flags out_of_date by diffing copy vs library
- POST /adventures/{id}/scripts/{sid}/sync overwrites the copy's code,
  preserving enabled/position/script_state
- legacy copies with no link fall back to a name match, then adopt the link
- Play page shows a Sync button only when a copy differs from its library

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TEpWMjnfPqzZ13nPMoGhs5

backend/app/schemas.py

@@ -237,6 +237,9 @@ class AdventureScriptOut(ORMModel):
     input_js: str
     context_js: str
     output_js: str
+    # Set by the router (not stored): True when a syncable library version
+    # exists whose code differs from this copy; None when nothing to sync.
+    out_of_date: bool | None = None
 
 
 class AdventureScriptUpdate(BaseModel):