Stage 4: alert acknowledgement and comments

- Migration 004: acknowledged_by/acknowledged_at columns on alerts,
  alert_comments table (FK cascade on delete)
- POST /api/alerts/{id}/acknowledge — stamps authed user + timestamp,
  returns updated alert with acknowledged_by username
- DELETE /api/alerts/{id}/acknowledge — clears ack (204)
- GET /api/alerts/{id}/comments — list in chronological order
- POST /api/alerts/{id}/comments — add comment (returns 201)
- DELETE /api/alerts/{id}/comments/{commentID} — own comments only (204)
- All alert queries now LEFT JOIN users for ack username

internal/db/migrations/004_alert_ack_comments.sql

@@ -0,0 +1,12 @@
+ALTER TABLE alerts ADD COLUMN acknowledged_by INTEGER REFERENCES users(id) ON DELETE SET NULL;
+ALTER TABLE alerts ADD COLUMN acknowledged_at INTEGER;
+
+CREATE TABLE alert_comments (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    alert_id   INTEGER NOT NULL REFERENCES alerts(id)  ON DELETE CASCADE,
+    user_id    INTEGER NOT NULL REFERENCES users(id)   ON DELETE CASCADE,
+    content    TEXT    NOT NULL,
+    created_at INTEGER NOT NULL DEFAULT (strftime('%s', 'now'))
+);
+
+CREATE INDEX alert_comments_alert_id_idx ON alert_comments(alert_id);