Feedback inbox: read/unread + delete

Make the admin Feedback section a real inbox.

* DB: feedback.read_at column (schema + idempotent migration).
* API: feedback list returns read_at; POST /api/admin/feedback/{id}/read
  {read} toggles it; DELETE /api/admin/feedback/{id} removes a message
  (both admin-gated). admin_stats gains feedback_unread; the Attention strip
  and the tab badge now count UNREAD, not total.
* Frontend: unread messages are highlighted with an accent rail + dot; an
  Unread filter joins the category chips; each message has Mark read/unread
  and Delete (confirm), with optimistic updates that revert on failure.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_dashboard.py

@@ -57,12 +57,12 @@ def test_attention_flags_resting_flagged_and_brief():
         {"failures": 0, "review_flag": 1},
         {"failures": 0, "review_flag": 0},
     ]
-    items = queries._attention(content, sources, feedback_7d=2)
+    items = queries._attention(content, sources, feedback_unread=2)
     texts = " | ".join(i["text"] for i in items)
     assert "1 source backing off" in texts          # one resting
     assert "1 source flagged" in texts               # one flagged
     assert "brief has only 5" in texts               # brief < 7
-    assert "2 feedback" in texts                      # recent feedback
+    assert "2 unread feedback" in texts              # unread feedback
     # 90/100 = 90% image coverage → no coverage warning
     assert "Image coverage" not in texts
 
@@ -71,4 +71,4 @@ def test_attention_clear_when_healthy():
     from goodnews import queries
     content = {"served": 100, "with_image": 95, "latest_brief_size": 7}
     sources = [{"failures": 0, "review_flag": 0}]
-    assert queries._attention(content, sources, feedback_7d=0) == []
+    assert queries._attention(content, sources, feedback_unread=0) == []