MediaRole auto-detection + manual role API + sync wiring (#7)

role_detector: classify items as Interstitial by collection name/tag patterns.
Wire chapter extraction + role detection into sync adapter (worker + presentation).
SQLite: persist/read role column, migration.
PUT /library/items/{id}/role endpoint for manual override.

k-tv-backend/migrations_sqlite/20260712000003_add_role_to_library_items.sql

@@ -0,0 +1 @@
+ALTER TABLE library_items ADD COLUMN role TEXT NOT NULL DEFAULT 'program';