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.up.sql

@@ -0,0 +1,14 @@
+-- Kanban: composite index for active task queries by residence with due date ordering
+CREATE INDEX IF NOT EXISTS idx_task_kanban_query
+    ON task_task (residence_id, is_cancelled, is_archived, next_due_date, due_date)
+    WHERE is_cancelled = false AND is_archived = false;
+
+-- Notifications: index for unread count (hot query)
+CREATE INDEX IF NOT EXISTS idx_notification_user_unread
+    ON notifications_notification (user_id, read)
+    WHERE read = false;
+
+-- Documents: residence + active filter
+CREATE INDEX IF NOT EXISTS idx_document_residence_active
+    ON documents_document (residence_id, is_active)
+    WHERE is_active = true;