Final cleanup: Update remaining comments and variable names

Changes:
- Update comments: "Bookmann UUID" → "Bookhoard UUID"
- Rename sidecar struct field: Bookmann → Bookhoard
- Update type names: SidecarBookmannConfig → SidecarBookhoardConfig
- Fix test database name in queue_test.go
- Fix uppercase env var examples in KOBO_SETUP.md

Internal Go variable names (BookmannUuid, bookmannUUID) left unchanged
as they're implementation details that don't affect functionality.

Part of project rename to Bookhoard.

internal/sync/queue_test.go

@@ -113,7 +113,7 @@ func TestPriorityConstants(t *testing.T) {
 func setupTestDB(t *testing.T) *database.Queries {
 	ctx := context.Background()
 
-	dbURL := "postgresql://postgres:postgres@localhost:5432/bookmann?sslmode=disable"
+	dbURL := "postgresql://postgres:postgres@localhost:5432/bookhoard?sslmode=disable"
 	dbPool, err := pgxpool.New(ctx, dbURL)
 	require.NoError(t, err, "Failed to connect to test database")