feat: add email verification code for registration and optimize email service

- Add POST /user/email/code endpoint to send 6-digit verification code
- Require email code verification before completing registration
- Add email code cache with 10min TTL and 5/day send rate limit
- Fix nil client guard, TLS conn leak, domain parsing, and Resend error body in email pkg
- Deploy via ssh inline command using current branch

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

deploy_dev.sh

@@ -1,13 +1,15 @@
 #!/bin/bash
 
-git push origin test
+BRANCH=$(git rev-parse --abbrev-ref HEAD)
 
-ssh ubuntu << 'ENDSSH'
-cd /home/yoge/Dev/doc_ai_backed
-git checkout test
-git pull origin test
-docker compose -f docker-compose.infra.yml up -d
-docker compose down
-docker image rm doc_ai_backed-doc_ai:latest
-docker compose up -d
-ENDSSH
+git push origin "$BRANCH"
+
+ssh ubuntu "
+  cd /home/yoge/Dev/doc_ai_backed &&
+  git checkout $BRANCH &&
+  git pull origin $BRANCH &&
+  docker compose -f docker-compose.infra.yml up -d &&
+  docker compose down &&
+  docker image rm doc_ai_backed-doc_ai:latest &&
+  docker compose up -d
+"