ci: add CI/CD pipeline with build, test, and deploy stages

[madhyamakist]

Summary

Adds GitHub Actions CI/CD pipeline for hashtaglocal-backend.

What's included:

• build-and-test job: Java 25 + PostGIS 16 service container, Gradle cache, spotlessCheck + JUnit tests, JAR artifact upload
• deploy-staging job: SCP JAR to staging-instance, systemctl restart, health check (triggers on push to staging)
• deploy-production job: SCP JAR to hashtaglocal-backend-prod, systemctl restart, health check (triggers on push to main)
• CI_CD.md: Documentation for pipeline setup, secrets, and troubleshooting

Required secrets:

• GCP_SA_KEY, GCP_PROJECT
• GCP_VM_INSTANCE_NAME_STAGING, GCP_ZONE_STAGING
• GCP_VM_INSTANCE_NAME_PROD, GCP_ZONE_PROD