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.

cmd/server/main.go

@@ -1,14 +1,14 @@
 package main
 
 import (
-	"bookmann/internal/config"
-	"bookmann/internal/database"
-	"bookmann/internal/handlers"
-	"bookmann/internal/middleware"
-	ratelimit "bookmann/internal/middleware"
-	"bookmann/internal/services"
-	"bookmann/internal/sync"
-	"bookmann/templates"
+	"bookhoard/internal/config"
+	"bookhoard/internal/database"
+	"bookhoard/internal/handlers"
+	"bookhoard/internal/middleware"
+	ratelimit "bookhoard/internal/middleware"
+	"bookhoard/internal/services"
+	"bookhoard/internal/sync"
+	"bookhoard/templates"
 	"bytes"
 	"context"
 	"log"