Add issue attachments

- IssueAttachment link table (issue <-> file) + migration
- GET/POST /issues/:issueId/attachments, DELETE .../attachments/:fileId
- Only own uploaded files can be attached; attach is idempotent
- Registers issues as a file owner: attached files stay visible to issue
  participants and are not collected by GC
- IssueDetail.attachments GraphQL field via @ResolveField, issue module untouched

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

prisma/migrations/20261002050156_issue_attachment/migration.sql

@@ -0,0 +1,18 @@
+-- CreateTable
+CREATE TABLE "issue_attachment" (
+    "issueId" INTEGER NOT NULL,
+    "fileId" TEXT NOT NULL,
+    "createdAt" TIMESTAMPTZ(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "issue_attachment_pkey" PRIMARY KEY ("issueId","fileId")
+);
+
+-- CreateIndex
+CREATE INDEX "issue_attachment_fileId_idx" ON "issue_attachment"("fileId");
+
+-- AddForeignKey
+ALTER TABLE "issue_attachment" ADD CONSTRAINT "issue_attachment_issueId_fkey" FOREIGN KEY ("issueId") REFERENCES "issue"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "issue_attachment" ADD CONSTRAINT "issue_attachment_fileId_fkey" FOREIGN KEY ("fileId") REFERENCES "file"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+