feat: Stage 3 — alert detail, acknowledge, comments, stats charts

Press enter on any alert to open a full-screen detail pane (viewport).
Keybindings in detail mode:
  a/A   acknowledge / unacknowledge (updates immediately)
  c     compose a comment (text input at bottom)
  [/]   cycle comment cursor up/down
  d     delete selected comment (y/N confirmation)
  s     assign placeholder — shows "not yet supported by server"
  S     open stats view: top alerts + by-hour/by-day ASCII bar charts
  esc   back to dashboard

API additions: GetAlert, AcknowledgeAlert, UnacknowledgeAlert,
GetComments, AddComment, DeleteComment, GetTopAlerts,
GetStatsByHour, GetStatsByDay.

internal/api/types.go

@@ -23,3 +23,28 @@ type AlertStats struct {
 	Firing   int `json:"firing"`
 	Resolved int `json:"resolved"`
 }
+
+type Comment struct {
+	ID        int64     `json:"id"`
+	AlertID   int64     `json:"alert_id"`
+	UserID    int64     `json:"user_id"`
+	Username  string    `json:"username"`
+	Content   string    `json:"content"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
+type TopAlert struct {
+	Name  string `json:"name"`
+	Count int    `json:"count"`
+}
+
+type HourStat struct {
+	Hour  int `json:"hour"`
+	Count int `json:"count"`
+}
+
+type DayStat struct {
+	Day     int    `json:"day"`
+	DayName string `json:"day_name"`
+	Count   int    `json:"count"`
+}