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>

artifacts/migrations/0016_artifact_deleted_at.py

@@ -0,0 +1,18 @@
+# Generated by Django 5.2.15 on 2026-09-07 11:32
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('artifacts', '0015_migrate_tokens_to_apikeys'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='artifact',
+            name='deleted_at',
+            field=models.DateTimeField(blank=True, db_index=True, null=True),
+        ),
+    ]