feat: Stage 2 — alert dashboard with live table and stats

Adds a bubbles/table alert list showing Name, Status, Started (relative
time), and Ack By columns. Stats bar shows total/firing/resolved counts.
Filter cycles firing → resolved → all with [f]. Auto-refresh fires on
the configured interval via tea.Tick. Table scrolls with j/k.

internal/api/types.go

@@ -0,0 +1,25 @@
+package api
+
+import "time"
+
+type Alert struct {
+	ID               int64             `json:"id"`
+	Fingerprint      string            `json:"fingerprint"`
+	Name             string            `json:"name"`
+	Status           string            `json:"status"`
+	Labels           map[string]string `json:"labels"`
+	Annotations      map[string]string `json:"annotations"`
+	StartsAt         time.Time         `json:"starts_at"`
+	EndsAt           *time.Time        `json:"ends_at"`
+	GeneratorURL     string            `json:"generator_url"`
+	ReceivedAt       time.Time         `json:"received_at"`
+	AcknowledgedByID *int64            `json:"acknowledged_by_id"`
+	AcknowledgedBy   string            `json:"acknowledged_by"`
+	AcknowledgedAt   *time.Time        `json:"acknowledged_at"`
+}
+
+type AlertStats struct {
+	Total    int `json:"total"`
+	Firing   int `json:"firing"`
+	Resolved int `json:"resolved"`
+}