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/migrations.py

@@ -71,6 +71,10 @@ MIGRATIONS: list[tuple[int, str]] = [
     (21, "ALTER TABLE settings ADD COLUMN user_id INTEGER REFERENCES users(id) ON DELETE CASCADE"),
     (22, "UPDATE settings SET user_id = 1"),
     (23, "CREATE UNIQUE INDEX IF NOT EXISTS ix_settings_user_id ON settings (user_id)"),
+    # Link each adventure-script copy back to its library Script so it can be
+    # 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"),
 ]
 
 LATEST_VERSION = max((v for v, _ in MIGRATIONS), default=1)