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

templates/app_new.html

@@ -117,6 +117,19 @@
                 >
             </div>
 
+            <div class="form-group">
+                <label for="healthcheck_command" class="label">Health Check Command</label>
+                <input
+                    type="text"
+                    id="healthcheck_command"
+                    name="healthcheck_command"
+                    value="{{.HealthcheckCommand}}"
+                    class="input font-mono"
+                    placeholder="curl -f http://localhost:8080/healthz || exit 1"
+                >
+                <p class="text-sm text-gray-500 mt-1">Custom shell command to check container health. Leave empty to use the image's default health check.</p>
+            </div>
+
             <div class="flex justify-end gap-3 pt-4">
                 <a href="/" class="btn-secondary">Cancel</a>
                 <button type="submit" class="btn-primary">Create App</button>