feat: Remove global db.Dbpool with dependency injection (Phase 0)

- Add Database struct in internal/db/database.go with Pool, Ctx, and RunMigrations()
- Update server.go to use Database struct with NewServerInstance()
- Add backend.go with InitBackend(), BackendRepo(), BackendCtx(), BackendPool()
- Update music.go and sync.go to use BackendRepo() and BackendCtx() instead of db.Dbpool/db.Ctx
- Update token_handler.go to accept pool parameter
- Update routes.go to use s.db.Pool for middleware
- Update cmd/main.go to use NewServerInstance() and HTTPServer()
- Update test_helpers.go to initialize backend with test database
- Update test files to use backend.BackendPool() and backend.BackendCtx()

Benefits:
- Easier to mock database for unit tests
- Follows Go best practices (dependency injection)
- Better architecture with explicit dependencies
- RunMigrations() replaces old Migrate_db() function

Note: Global db.Dbpool and db.Ctx still exist in dbHelper.go for backward compatibility
with test_helpers.go, but production code no longer uses them.

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

internal/server/zz_music_handler_test.go

@@ -15,8 +15,8 @@ import (
 
 // ensureSyncRan ensures that sync has been run before testing music endpoints
 func ensureSyncRan(t *testing.T, e *echo.Echo) {
-	repo := repository.New(db.Dbpool)
-	games, err := repo.FindAllGames(db.Ctx)
+	repo := repository.New(backend.BackendPool())
+	games, err := repo.FindAllGames(backend.BackendCtx())
 	assert.NoError(t, err)
 
 	if len(games) == 0 {