Additive scene filenames endpoint (append without full replace)

[moToroTor]

Adds POST /api/scene/filenames/{scene-id}, which merges basenames into the scene's filenames array without touching any other field.

Motivation: the only write path today is POST /api/scene/edit/{scene-id}, which overwrites every field it receives. External tools (e.g. download managers that know the expected filename for a scene ahead of the next library scan) have no way to register a filename non-destructively — they must GET the full scene, merge, and POST it all back.

Behavior:

• Request body {"filenames": [...]}; new names are appended, pre-existing entries untouched.
• Idempotent: re-appending the same name is an exact-match no-op (same == comparison the scan matcher uses).
• Encoding goes through encoding/json, identical to the scan write path, so the LIKE matcher keeps matching.
• Audit + index: AddAction entry on change and a search-index refresh, same as an edit.
• Errors: missing key 400, unknown scene id 404, corrupt stored list 500 with no write.
• A package mutex serializes concurrent appends so read-modify-write cannot lose entries.

Includes pkg/api/scenes_filenames_test.go covering append, idempotency, and the error cases. No changes to scan, edit, or scrapers.