Add webhook logging, pagination, middleware, migrations, and prod hardening

- Webhook event logging repo and subscription webhook idempotency
- Pagination helper (echohelpers) with cursor/offset support
- Request ID and structured logging middleware
- Push client improvements (FCM HTTP v1, better error handling)
- Task model version column, business constraint migrations, targeted indexes
- Expanded categorization chain tests
- Email service and config hardening
- CI workflow updates, .gitignore additions, .env.example updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrations/015_targeted_indexes.down.sql

@@ -0,0 +1,3 @@
+DROP INDEX IF EXISTS idx_task_kanban_query;
+DROP INDEX IF EXISTS idx_notification_user_unread;
+DROP INDEX IF EXISTS idx_document_residence_active;