Rename backend code references: Bookmann → Bookhoard

Backend changes:
- Update import paths: bookmann/internal → bookhoard/internal
- Rename struct fields: BookmannUUID → BookhoardUUID
- Update handler function names: mapContentIdToBookmannUUID → mapContentIdToBookhoardUUID
- Update HTTP response headers: X-Bookmann-* → X-Bookhoard-*
- Update service and middleware references
- Update main.go imports and references

This is part 2 of the project rename to Bookhoard.

internal/services/conversion_service.go

@@ -11,7 +11,7 @@ import (
 	"path/filepath"
 	"time"
 
-	"bookmann/internal/database"
+	"bookhoard/internal/database"
 	"github.com/jackc/pgx/v5/pgtype"
 )