feat: archive images reach the library: game builds stay whole, and re-extract covers every archive

Most archive attachments never had their images imported. The re-extract
action skipped archives without a post, and nothing kept game builds out
of an extraction.

- An archive holding executables, libraries, or game-engine data
  (Ren'Py, Unity, Godot, RPG Maker, Python builds) is kept as an
  attachment and not extracted. The same holds for one nested inside an
  art pack.
- Re-extract now reaches archives imported from a folder without a post.
  Their images go to the artist, in a folder named after the archive.
- Re-extract passes over archives whose members are already recorded.
- Re-extract stages each archive by symlink instead of copying it.
- The card says what the action does now.

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

frontend/src/components/settings/ArchiveReextractCard.vue

@@ -1,17 +1,15 @@
 <template>
-  <!-- #713: re-extract PostAttachments that are really archives but were filed
-       opaquely before the magic-byte gate, and attach their images to the post. -->
   <MaintenanceTile
     icon="mdi-folder-zip-outline"
     title="Re-extract archives"
-    blurb="Extract images from zip attachments filed opaquely."
+    blurb="Import the images inside archive attachments that were never extracted."
     :open="busy"
   >
       <p class="text-body-2 mb-3">
-        Some posts attached an archive (zip) whose images weren't extracted
-        because the downloaded file had no usable extension. This scans existing
-        attachments, extracts any that are really archives, and attaches their
-        images to the post. Idempotent — safe to run more than once.
+        Goes through every stored zip, rar and 7z, including ones imported from
+        a folder without a post, and imports the images inside. Archives already
+        extracted are passed over, and game or app builds stay whole. Safe to
+        run again.
       </p>
       <v-btn color="primary" rounded="pill" :loading="busy" @click="run">
         <v-icon start>mdi-folder-zip-outline</v-icon> Re-extract archives now