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/dashboard.rs

@@ -57,6 +57,7 @@ pub async fn serve(port: u16, coord: Arc<Coordinator>) -> Result<()> {
         .route("/api/state-file", get(get_state_file))
         .route("/api/reminders", get(api_reminders))
         .route("/cancel-reminder/{id}", post(post_cancel_reminder))
+        .route("/retry-reminder/{id}", post(post_retry_reminder))
         .route("/api/agent-config/{name}", get(get_agent_config))
         .route("/request-spawn", post(post_request_spawn))
         .route("/op-send", post(post_op_send))
@@ -1126,6 +1127,25 @@ async fn post_cancel_reminder(
     }
 }
 
+/// Reset a pending reminder's failure state so the scheduler
+/// retries it on the next tick. Useful when the failure was
+/// transient (sqlite lock contention, disk full → freed up) and
+/// the operator wants delivery to resume immediately instead of
+/// the row sitting in attempt-count-capped purgatory.
+async fn post_retry_reminder(
+    State(state): State<AppState>,
+    AxumPath(id): AxumPath<i64>,
+) -> Response {
+    match state.coord.broker.reset_reminder_failure(id) {
+        Ok(0) => error_response(&format!("reminder {id} not pending (already delivered?)")),
+        Ok(_) => {
+            tracing::info!(%id, "operator reset reminder failure for retry");
+            (StatusCode::OK, "ok").into_response()
+        }
+        Err(e) => error_response(&format!("retry reminder {id} failed: {e:#}")),
+    }
+}
+
 async fn post_purge_tombstone(
     State(state): State<AppState>,
     AxumPath(name): AxumPath<String>,