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/handlers/auth.go

@@ -1,8 +1,8 @@
 package handlers
 
 import (
-	"bookmann/internal/database"
-	"bookmann/internal/middleware"
+	"bookhoard/internal/database"
+	"bookhoard/internal/middleware"
 	"fmt"
 	"net/http"
 	"path/filepath"