Add new part type 'abspann' (credits/Abspann presentation) to the macro
assignment enum, validation and UI.
- Relax part_type from enum to plain string on macro_assignments,
service_macro_overrides and service_macro_assignments so 'abspann' (and
any controller-validated value) is accepted across MySQL and SQLite.
- Add 'abspann' to Rule::in validation in MacroAssignmentController and
ServiceMacroOverrideController.
- Include 'abspann' in the per-service macros_per_part list in
ServiceController so the override UI exposes it.
- Add 'abspann' to the parts array in Settings/MacroAssignments.vue
(by_label stays song-only; abspann offers all/first/last slide).
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