reminders: persist + surface delivery failures

Broker schema gains attempt_count INTEGER + last_error TEXT
columns via idempotent ALTER TABLE migration (pragma-probed so
fresh + existing dbs converge). reminder_scheduler::tick calls
record_reminder_failure on every deliver_reminder error,
bumping the counter + stashing the message. get_due_reminders
filters out rows where attempt_count >= MAX_REMINDER_ATTEMPTS
(5) so the scheduler stops retrying a stuck row until the
operator intervenes.

new POST /retry-reminder/{id} → reset_reminder_failure clears
the counters; next 5s tick re-attempts. cancel-reminder
unchanged (hard-delete).

dashboard renders failed rows with a red left rule, the error
text inline, and a ⚠ N failed badge. ↻ R3TRY button appears
when attempt_count > 0 — sits next to ✗ C4NC3L in a small
actions row below the body.

hive-c0re/src/reminder_scheduler.rs

@@ -71,12 +71,24 @@ fn tick(coord: &Arc<Coordinator>) {
     for (agent, id, message, file_path) in due {
         let body = prepare_body(&agent, &message, file_path.as_deref());
         if let Err(e) = coord.broker.deliver_reminder(id, &agent, &body) {
+            let reason = format!("{e:#}");
             tracing::warn!(
                 reminder_id = id,
                 %agent,
-                error = ?e,
+                error = %reason,
                 "failed to deliver reminder"
             );
+            // Persist the failure so the dashboard can surface it +
+            // bump attempt_count. After MAX_REMINDER_ATTEMPTS the
+            // row drops out of `get_due_reminders` and waits for
+            // operator retry / cancel.
+            if let Err(persist_err) = coord.broker.record_reminder_failure(id, &reason) {
+                tracing::warn!(
+                    reminder_id = id,
+                    error = ?persist_err,
+                    "failed to persist reminder failure"
+                );
+            }
         }
     }
 }