v0.5.0: Trash (soft-delete) + Keep folder

Delete now moves an artifact to a recoverable Trash instead of hard-removing.

- Model: Artifact.deleted_at (nullable, indexed) = in Trash.
- Soft-delete everywhere: DELETE /api/artifacts/{id} sets deleted_at
  (idempotent 204); session POST /a/{id}/delete for the feed's hover Delete
  icon; POST /a/{id}/undelete restores; POST /feed/empty-trash purges.
- Trashed artifacts are off-quota (deleting frees quota) and eviction-protected
  (never evicted early); hidden from feed, sources, counts, and all API reads
  (list/detail/raw).
- Sidebar: "Deleted" and "Keep" system folders at the bottom (counts, hidden
  when empty); ?folder=deleted / ?folder=keep views reuse the row rendering.
  Deleted view has hover Undelete + an Empty Trash button. Keep = view over
  is_kept. Live-poll disabled in folder views.
- Docs (api.md delete section, CLAUDE.md), 10 new/updated tests. 150 passing;
  API soft-delete lifecycle verified live.

Purge is manual (Empty Trash) this release; auto purge-on-delete (background,
~3-day retention) is the planned fast-follow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/artifacts/_artifact_rows.html

@@ -30,5 +30,10 @@
       {% endif %}
     </span>
     <span class="artifact-source">{{ artifact.source }}</span>
+    {% if in_trash %}
+      <span class="row-action row-undelete" role="button" tabindex="0" data-undelete="{{ artifact.id }}" title="Restore from Trash" aria-label="Restore {{ artifact.title }}">&#8617;</span>
+    {% else %}
+      <span class="row-action row-delete" role="button" tabindex="0" data-delete="{{ artifact.id }}" title="Delete" aria-label="Delete {{ artifact.title }}">&#128465;</span>
+    {% endif %}
   </a>
 {% endfor %}