Stage 3: Alertmanager webhook ingestion and alert query API

- Migration 003: alerts table with fingerprint UNIQUE, JSON label/annotation
  columns, nullable ends_at, and indexed status/name/received_at
- POST /api/alertmanager/webhook — upserts each alert by fingerprint;
  zero endsAt ("0001-01-01") stored as NULL (still firing)
- GET /api/alerts — filtered list (?status, ?name, ?from, ?to, ?limit)
- GET /api/alerts/{id} — single alert lookup

internal/models/alert.go

@@ -0,0 +1,16 @@
+package models
+
+import "time"
+
+type Alert struct {
+	ID           int64             `json:"id"`
+	Fingerprint  string            `json:"fingerprint"`
+	Name         string            `json:"name"`
+	Status       string            `json:"status"` // "firing" or "resolved"
+	Labels       map[string]string `json:"labels"`
+	Annotations  map[string]string `json:"annotations"`
+	StartsAt     time.Time         `json:"starts_at"`
+	EndsAt       *time.Time        `json:"ends_at,omitempty"`
+	GeneratorURL string            `json:"generator_url"`
+	ReceivedAt   time.Time         `json:"received_at"`
+}