T20: Song DB Page
- Songs/Index.vue with search, action buttons, pagination
- Upload area for .pro files (calls T23 placeholder)
- Song-Datenbank nav link added to AuthenticatedLayout
- Tests: 9 new (44 assertions)
T21: Song DB Edit Popup
- SongEditModal.vue with metadata + ArrangementConfigurator
- Auto-save with fetch (500ms debounce for text, immediate on blur)
- Tests: 11 new (53 assertions)
T22: Song DB Translate Page
- Songs/Translate.vue with two-column editor
- URL fetch or manual paste, line-count constraints
- Group headers with colors, save marks has_translation=true
- Tests: 1 new (12 assertions)
T23: .pro File Placeholders
- ProParserNotImplementedException with HTTP 501
- ProFileController with importPro/downloadPro placeholders
- German error messages
- Tests: 5 new (7 assertions)
T24: Service Finalization + Status
- Two-step finalization with warnings (unmatched songs, missing slides)
- Download placeholder toast
- isReadyToFinalize accessor on Service model
- Tests: 11 new (30 assertions)
All tests passing: 174/174 (905 assertions)
Build: ✓ Vite production build successful
German UI: All user-facing text in German with 'Du' form
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