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/book_matching.go

@@ -1,8 +1,8 @@
 package handlers
 
 import (
-	"bookmann/internal/database"
-	"bookmann/internal/services"
+	"bookhoard/internal/database"
+	"bookhoard/internal/services"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -294,7 +294,7 @@ func (h *Handler) AutoLinkBooks(c echo.Context) error {
 			bestMatch := match.Matches[0]
 
 			_, err := h.db.CreateDeviceFileAlias(ctx, database.CreateDeviceFileAliasParams{
-				MediaItemID:     pgtype.UUID{Bytes: bestMatch.BookmannUUID, Valid: true},
+				MediaItemID:     pgtype.UUID{Bytes: bestMatch.BookhoardUUID, Valid: true},
 				DeviceID:        book.DeviceID,
 				FilePath:        book.FilePath.String,
 				FileSha256:      pgtype.Text{String: "", Valid: false},
@@ -304,14 +304,14 @@ func (h *Handler) AutoLinkBooks(c echo.Context) error {
 			if err == nil {
 				h.db.LinkUnlinkedBook(ctx, database.LinkUnlinkedBookParams{
 					ID:              book.ID,
-					MediaItemID:     pgtype.UUID{Bytes: bestMatch.BookmannUUID, Valid: true},
+					MediaItemID:     pgtype.UUID{Bytes: bestMatch.BookhoardUUID, Valid: true},
 					ConfidenceScore: toFloat8(bestMatch.Confidence),
 				})
 
 				results = append(results, map[string]interface{}{
 					"unlinked_book_id":      uuid.UUID(book.ID.Bytes).String(),
 					"title":                 book.Title.String,
-					"matched_media_item_id": bestMatch.BookmannUUID.String(),
+					"matched_media_item_id": bestMatch.BookhoardUUID.String(),
 					"confidence":            bestMatch.Confidence,
 					"match_method":          bestMatch.MatchMethod,
 				})