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/000013_business_constraints.up.sql

@@ -0,0 +1,19 @@
+-- Prevent task from being both cancelled and archived simultaneously
+ALTER TABLE task_task ADD CONSTRAINT chk_task_not_cancelled_and_archived
+    CHECK (NOT (is_cancelled = true AND is_archived = true));
+
+-- Subscription tier must be a valid value
+ALTER TABLE subscriptions_usersubscription ADD CONSTRAINT chk_subscription_tier
+    CHECK (tier IN ('free', 'pro'));
+
+-- Notification: sent_at must be set when sent is true
+ALTER TABLE notifications_notification ADD CONSTRAINT chk_notification_sent_consistency
+    CHECK ((sent = false) OR (sent = true AND sent_at IS NOT NULL));
+
+-- One subscription per user
+ALTER TABLE subscriptions_usersubscription ADD CONSTRAINT uq_subscription_user
+    UNIQUE (user_id);
+
+-- One notification preference per user
+ALTER TABLE notifications_notificationpreference ADD CONSTRAINT uq_notif_pref_user
+    UNIQUE (user_id);