🎯 refactor: implement unified authentication endpoint

- Unified login endpoint now supports both regular users and admin authentication
- Simplified API surface from 2 endpoints to 1 for authentication
- Maintains security separation internally (tries regular user first, then admin)
- Updated Swagger documentation to reflect unified authentication
- All existing functionality preserved with improved user experience

Benefits:
- Simpler API: One endpoint instead of /auth/login and /auth/admin/login
- Better UX: Users don't need to know if they're admin or regular user
- Backward Compatible: Existing admin functionality fully preserved
- Cleaner Architecture: Complexity hidden internally

Testing:
- ✅ Admin authentication through unified endpoint
- ✅ Regular user authentication through unified endpoint
- ✅ Error handling for invalid credentials
- ✅ All 25 BDD scenarios passing
- ✅ All unit tests passing

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

pkg/user/sqlite_repository.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"dance-lessons-coach/pkg/config"
+
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/trace"