feat(diagnostics): harden ingestion and delivery

services/diagnostics-api/migrations/0002_add_bug_occurred_at.sql

@@ -0,0 +1,4 @@
+ALTER TABLE bugs ADD COLUMN occurred_at INTEGER;
+
+-- Existing reports predate this field; their receipt time is the best available value.
+UPDATE bugs SET occurred_at = received_at WHERE occurred_at IS NULL;