prod: UI Optimisée mise en production

- Documentation archivée et réorganisée
- Backend: Ajout tests, migrations, library service, rate limiting
- Frontend: Suppression Flutter, focus sur interface web HTML/JS
- Tailwind CSS ajouté pour le style
- Améliorations UX et corrections bugs

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via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

backend/fix_completed_column.sql

@@ -0,0 +1,5 @@
+-- Fix completed column type
+ALTER TABLE listening_history ALTER COLUMN completed TYPE BOOLEAN USING CASE WHEN completed = 1 THEN TRUE ELSE FALSE END;
+
+-- Add comment
+COMMENT ON COLUMN listening_history.completed IS 'Whether the track was listened to completion';