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_test.go

@@ -120,7 +120,7 @@ func TestEntrySetIdentifier(t *testing.T) {
 		t.Fatal("expected Identifier to be set, but it was nil")
 	}
 
-	if entry.Identifier.ID != "bookmann" {
+	if entry.Identifier.ID != "bookhoard" {
 		t.Errorf("expected Identifier ID to be 'bookmann', got '%s'", entry.Identifier.ID)
 	}
 
@@ -131,14 +131,14 @@ func TestEntrySetIdentifier(t *testing.T) {
 
 func TestEntryAddMetadata(t *testing.T) {
 	entry := NewEntry("urn:uuid:test-id", "Test Title", "Test Author", "2023-01-01T00:00:00Z")
-	entry.AddMetadata("bookmann:sha256", "abc123...")
+	entry.AddMetadata("bookhoard:sha256", "abc123...")
 
 	if len(entry.Metadata) != 1 {
 		t.Fatalf("expected 1 metadata item, got %d", len(entry.Metadata))
 	}
 
 	meta := entry.Metadata[0]
-	if meta.Property != "bookmann:sha256" {
+	if meta.Property != "bookhoard:sha256" {
 		t.Errorf("expected Property to be 'bookmann:sha256', got '%s'", meta.Property)
 	}
 
@@ -181,7 +181,7 @@ func TestFeedGenerateXML(t *testing.T) {
 	entry := NewEntry("urn:uuid:book-id", "Test Book", "Test Author", "2023-01-01T00:00:00Z")
 	entry.AddAcquisitionLink("http://example.com/book.epub", "application/epub+zip")
 	entry.SetIdentifier("book-uuid-123")
-	entry.AddMetadata("bookmann:sha256", "abc123...")
+	entry.AddMetadata("bookhoard:sha256", "abc123...")
 	feed.AddEntry(entry)
 
 	output, err := feed.GenerateXML()
@@ -205,8 +205,8 @@ func TestFeedGenerateXML(t *testing.T) {
 		`<dc:creator>Test Author</dc:creator>`,
 		`<link href="http://example.com/book.epub"`,
 		`rel="http://opds-spec.org/acquisition/open-access"`,
-		`<dc:identifier id="bookmann">book-uuid-123</dc:identifier>`,
-		`<meta property="bookmann:sha256">abc123...</meta>`,
+		`<dc:identifier id="bookhoard">book-uuid-123</dc:identifier>`,
+		`<meta property="bookhoard:sha256">abc123...</meta>`,
 	}
 
 	for _, required := range requiredStrings {