feat: Last Seen column tracking Alertmanager re-send heartbeat

The alert list showed only Started, which comes from Prometheus and
never changes for the lifetime of an alert instance. A firing alert
that started 12 days ago looked identical whether Alertmanager
refreshed it 30 seconds ago or went silent a week ago.

terdut-server already tracks this: the webhook upsert sets
received_at on every accepted payload, including the periodic
re-sends issued at repeat_interval, and its archiver treats the
field as a liveness heartbeat. The field was already decoded into
api.Alert.ReceivedAt and simply never rendered.

Add a Last Seen column to the alert tables, rendered with the
existing humanAgo helper. The Alerts and Archived tabs share
alertColumns/alertRows, so both pick it up. The width budget is
re-derived for five columns; the slack constant now accounts for
all of bubbles' per-cell padding, so the table lands exactly on
the terminal width instead of overflowing by two columns as it
did with four.

The detail view gains a matching Last Seen line, with the timeline
labels widened to keep values aligned. Since received_at stops
advancing once an alert resolves, also pull through the server's
resolution_source and show it in the status header
(RESOLVED · alertmanager vs RESOLVED · expiry) so a frozen
timestamp is explained.

internal/api/types.go

@@ -17,6 +17,11 @@ type Alert struct {
 	AcknowledgedBy   string            `json:"acknowledged_by"`
 	AcknowledgedAt   *time.Time        `json:"acknowledged_at"`
 	ArchivedAt       *time.Time        `json:"archived_at,omitempty"`
+
+	// ResolutionSource records why a resolved alert left the firing state:
+	// "alertmanager" for a real resolved webhook, "expiry" when the server
+	// inferred it after the alert stopped being refreshed.
+	ResolutionSource *string `json:"resolution_source,omitempty"`
 }
 
 type AlertStats struct {