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/opds/feed.go

@@ -125,7 +125,7 @@ func (e *Entry) AddLink(href, linkType, rel string) {
 // SetIdentifier sets the canonical identifier
 func (e *Entry) SetIdentifier(id string) {
 	e.Identifier = &Identifier{
-		ID:      "bookmann",
+		ID:      "bookhoard",
 		Content: id,
 	}
 }