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/000012_webhook_event_log.up.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS webhook_event_log (
+    id SERIAL PRIMARY KEY,
+    event_id VARCHAR(255) NOT NULL,
+    provider VARCHAR(20) NOT NULL,
+    event_type VARCHAR(100) NOT NULL,
+    processed_at TIMESTAMPTZ DEFAULT NOW(),
+    payload_hash VARCHAR(64),
+    UNIQUE(provider, event_id)
+);