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/models.py

@@ -60,6 +60,9 @@ class Artifact(models.Model):
     metadata = models.JSONField(default=dict, blank=True)
     read_at = models.DateTimeField(null=True, blank=True)
     is_kept = models.BooleanField(default=False)
+    # Soft-delete: non-null = in Trash (hidden from feed/sources, off-quota,
+    # recoverable via undelete, purged by Empty Trash). NULL = live.
+    deleted_at = models.DateTimeField(null=True, blank=True, db_index=True)
     size_bytes = models.BigIntegerField(default=0)
     # NULL = not shared. Many rows share NULL (SQL treats each NULL as
     # distinct, so `unique=True` still holds); a real value must be unique
@@ -74,6 +77,10 @@ class Artifact(models.Model):
     def is_shared(self):
         return bool(self.share_token)
 
+    @property
+    def is_deleted(self):
+        return self.deleted_at is not None
+
     class Meta:
         ordering = ["-created_at"]