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/db/migrations/003_alerts.sql

@@ -0,0 +1,16 @@
+CREATE TABLE alerts (
+    id            INTEGER PRIMARY KEY AUTOINCREMENT,
+    fingerprint   TEXT    NOT NULL UNIQUE,
+    name          TEXT    NOT NULL,
+    status        TEXT    NOT NULL CHECK(status IN ('firing', 'resolved')),
+    labels        TEXT    NOT NULL DEFAULT '{}',
+    annotations   TEXT    NOT NULL DEFAULT '{}',
+    starts_at     INTEGER NOT NULL,
+    ends_at       INTEGER,
+    generator_url TEXT    NOT NULL DEFAULT '',
+    received_at   INTEGER NOT NULL DEFAULT (strftime('%s', 'now'))
+);
+
+CREATE INDEX alerts_status_idx     ON alerts(status);
+CREATE INDEX alerts_name_idx       ON alerts(name);
+CREATE INDEX alerts_received_at_idx ON alerts(received_at DESC);