admin: read/unread triage for load errors (unread by default, mark read/all)

The load-error log had no way to clear reviewed entries. Add a read_at column to
client_errors and a read/unread model mirroring the feedback inbox:
- GET /api/admin/client-errors?show=unread|read|all (default unread; returns id+read)
- POST /api/admin/client-errors/read-all  (mark all unread read)
- POST /api/admin/client-errors/{id}/read {read: bool}  (per-row toggle)
Headline stat is now "Unread load errors" (admin_stats.client_errors.unread), so the
red badge clears as you triage. Admin UI: Unread/Read/All tabs, a "Mark all read"
button, and a per-row ✓/↩ toggle; reading an entry drops it from the default view.
14-day auto-prune still bounds the table. Tests cover filter, toggle, mark-all,
404, and gating.

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

goodnews/db.py

@@ -651,6 +651,11 @@ def _migrate(conn: sqlite3.Connection) -> None:
     if fb_cols and "read_at" not in fb_cols:
         conn.execute("ALTER TABLE feedback ADD COLUMN read_at TEXT")
 
+    # client_errors.read_at — admin triages load-error beacons (default view = unread).
+    ce_cols = {row["name"] for row in conn.execute("PRAGMA table_info(client_errors)")}
+    if ce_cols and "read_at" not in ce_cols:
+        conn.execute("ALTER TABLE client_errors ADD COLUMN read_at TEXT")
+
     # feedback_replies.message_html (rendered Markdown subset) added later.
     rep_cols = {row["name"] for row in conn.execute("PRAGMA table_info(feedback_replies)")}
     if rep_cols and "message_html" not in rep_cols: