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>

static/agentfeed.css

@@ -1055,6 +1055,57 @@ kbd {
   animation: agentfeed-row-arrive 1.5s ease-out;
 }
 
+/* Hover/focus row actions (delete in the feed, undelete in Trash) */
+.artifact-row {
+  position: relative;
+}
+
+.row-action {
+  position: absolute;
+  right: 0.5rem;
+  top: 50%;
+  transform: translateY(-50%);
+  opacity: 0;
+  cursor: pointer;
+  font-size: 0.95rem;
+  line-height: 1;
+  padding: 0.15rem 0.3rem;
+  border-radius: 4px;
+  background: var(--surface);
+  color: var(--muted);
+}
+
+.artifact-row:hover .row-action,
+.artifact-row:focus-within .row-action {
+  opacity: 1;
+}
+
+.row-action:hover {
+  background: var(--surface-alt);
+  color: var(--text);
+}
+
+.row-delete:hover {
+  color: var(--danger-text-plain);
+}
+
+/* System folders (Keep / Deleted) at the bottom of the sidebar */
+.source-folders {
+  border-top: 1px solid var(--grid-line);
+  margin-top: 0.4rem;
+  padding-top: 0.4rem;
+}
+
+.empty-trash-button {
+  background: var(--danger-bg);
+  border: 1px solid var(--danger-text);
+  border-radius: 6px;
+  color: var(--danger-text);
+  cursor: pointer;
+  font-size: 0.78rem;
+  padding: 0.2rem 0.55rem;
+}
+
 .api-key-create {
   align-items: end;
   display: flex;