Remove legacy key file support after DB migration

- SerieScanner: Remove key file fallback, keep data file fallback
- SystemSettings: Add legacy_key_cleanup_completed flag
- initialization_service: Add cleanup task to remove key files from folders with DB entries
- Tests updated to reflect key file removal from legacy path

Key files caused duplicate key errors on folder rename. DB is now sole source of truth.

src/server/database/models.py

@@ -617,6 +617,10 @@ class SystemSettings(Base, TimestampMixin):
         Boolean, nullable=False, default=False, server_default="0",
         doc="Whether legacy key/data file migration has been completed"
     )
+    legacy_key_cleanup_completed: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False, server_default="0",
+        doc="Whether legacy key file cleanup has been completed"
+    )
     last_scan_timestamp: Mapped[Optional[datetime]] = mapped_column(
         DateTime(timezone=True), nullable=True,
         doc="Timestamp of the last completed scan"