feat: add custom health check commands per app

Add configurable health check commands per app via a new
'healthcheck_command' field. When set, the command is passed
to Docker as a CMD-SHELL health check on the container.
When empty, the image's default health check is used.

Changes:
- Add migration 007 for healthcheck_command column on apps table
- Add HealthcheckCommand field to App model with full CRUD support
- Add buildHealthcheck() to docker client for CMD-SHELL config
- Pass health check command through CreateContainerOptions
- Add health check command input to app create/edit UI forms
- Extract optionalNullString helper to reduce handler complexity
- Update README features list

closes #81

README.md

@@ -8,7 +8,7 @@ A simple self-hosted PaaS that auto-deploys Docker containers from Git repositor
 - Per-app SSH keypairs for read-only deploy keys
 - Per-app UUID-based webhook URLs for Gitea integration
 - Branch filtering - only deploy on configured branch changes
-- Environment variables, labels, and volume mounts per app
+- Environment variables, labels, volume mounts, and custom health checks per app
 - CPU and memory resource limits per app
 - Docker builds via socket access
 - Notifications via ntfy and Slack-compatible webhooks