Add attachments to comments

- CommentAttachment link table (comment <-> file) + migration
- fileIds in create/update comment; update replaces the whole set,
  synced in one transaction with the comment
- CommentDto.attachments; empty for deleted comments
- Soft delete drops attachments so GC can collect the files
- Max 10 files per comment, ordered by upload time (uuid v7)
- Registers comments as a file owner
- Rename CommentAuthorExpanderService -> CommentExpanderService
  (expands authors and files)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

prisma/migrations/20261002060755_comment_attachment/migration.sql

@@ -0,0 +1,18 @@
+-- CreateTable
+CREATE TABLE "comment_attachment" (
+    "commentId" TEXT NOT NULL,
+    "fileId" TEXT NOT NULL,
+    "createdAt" TIMESTAMPTZ(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "comment_attachment_pkey" PRIMARY KEY ("commentId","fileId")
+);
+
+-- CreateIndex
+CREATE INDEX "comment_attachment_fileId_idx" ON "comment_attachment"("fileId");
+
+-- AddForeignKey
+ALTER TABLE "comment_attachment" ADD CONSTRAINT "comment_attachment_commentId_fkey" FOREIGN KEY ("commentId") REFERENCES "comment"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "comment_attachment" ADD CONSTRAINT "comment_attachment_fileId_fkey" FOREIGN KEY ("fileId") REFERENCES "file"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+