fix(documents): tighten archive/restore idempotency + document fire-and-forget

Three follow-ups from Task 6 code review:
1. applyEntityArchivedSuffix short-circuits when the folder is already
   archived — prevents archivedAt drift on backfill replay.
2. applyEntityRestoredSuffix short-circuits when the folder was never
   archived — matches the docstring's "no-op" claim.
3. Inline comment on archiveClient's fire-and-forget hook documents
   why Task 6 uses void (archive UI doesn't depend on folder sync)
   while Task 5 uses await (rename should be visible to the next
   read).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/services/document-folders.service.ts

@@ -600,6 +600,7 @@ export async function applyEntityArchivedSuffix(
   const newName = folder.name.endsWith(ARCHIVED_SUFFIX)
     ? folder.name
     : `${folder.name}${ARCHIVED_SUFFIX}`;
+  if (newName === folder.name && folder.archivedAt) return; // Already archived, no-op.
   await db
     .update(documentFolders)
     .set({ name: newName, archivedAt: new Date(), updatedAt: new Date() })
@@ -628,6 +629,7 @@ export async function applyEntityRestoredSuffix(
   const newName = folder.name.endsWith(ARCHIVED_SUFFIX)
     ? folder.name.slice(0, -ARCHIVED_SUFFIX.length)
     : folder.name;
+  if (newName === folder.name && !folder.archivedAt) return; // Wasn't archived, no-op.
   await db
     .update(documentFolders)
     .set({ name: newName, archivedAt: null, updatedAt: new Date() })