feat: detail page attachments, preview, and file metadata

Add file displayName/sortOrder APIs, result preview with Excel/video support,
unified attachment/link editing, feedback modal, and AuthProvider fix.
Run prisma migrate deploy on Render builds.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/prisma/migrations/20260605140000_file_display_name/migration.sql

@@ -0,0 +1 @@
+ALTER TABLE "files" ADD COLUMN IF NOT EXISTS "displayName" TEXT;

backend/prisma/migrations/20260605150000_file_sort_order/migration.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "files" ADD COLUMN IF NOT EXISTS "sortOrder" INTEGER NOT NULL DEFAULT 0;
+CREATE INDEX IF NOT EXISTS "files_milestoneId_sortOrder_idx" ON "files"("milestoneId", "sortOrder");

backend/prisma/schema.prisma

@@ -138,6 +138,8 @@ model File {
   milestoneId  String?
   filename     String   // 저장된 파일명 (UUID)
   originalName String   // 원본 파일명
+  displayName  String?  // 표시명 (없으면 originalName 사용)
+  sortOrder    Int      @default(0)
   mimetype     String
   size         Int
   path         String