feat: JAR retention policy with nightly cleanup job

Per-environment "keep last N versions" setting (default 5, null for
unlimited). Nightly scheduled job at 03:00 deletes old versions from
both database and disk, skipping any version that is currently deployed.

Full stack:
- V6 migration: adds jar_retention_count column to environments
- Environment record, repository, service, admin controller endpoint
- JarRetentionJob: @Scheduled nightly, iterates environments and apps
- UI: retention policy editor on admin Environments page with
  toggle between limited/unlimited and version count input
- AppVersionRepository.delete() for version cleanup

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-core/src/main/java/com/cameleer3/server/core/runtime/AppVersionRepository.java

@@ -9,4 +9,5 @@ public interface AppVersionRepository {
     Optional<AppVersion> findById(UUID id);
     int findMaxVersion(UUID appId);
     UUID create(UUID appId, int version, String jarPath, String jarChecksum, String jarFilename, Long jarSizeBytes);
+    void delete(UUID id);
 }

cameleer3-server-core/src/main/java/com/cameleer3/server/core/runtime/Environment.java

@@ -11,5 +11,6 @@ public record Environment(
         boolean production,
         boolean enabled,
         Map<String, Object> defaultContainerConfig,
+        Integer jarRetentionCount,
         Instant createdAt
 ) {}

cameleer3-server-core/src/main/java/com/cameleer3/server/core/runtime/EnvironmentRepository.java

@@ -12,5 +12,6 @@ public interface EnvironmentRepository {
     UUID create(String slug, String displayName, boolean production);
     void update(UUID id, String displayName, boolean production, boolean enabled);
     void updateDefaultContainerConfig(UUID id, Map<String, Object> defaultContainerConfig);
+    void updateJarRetentionCount(UUID id, Integer jarRetentionCount);
     void delete(UUID id);
 }

cameleer3-server-core/src/main/java/com/cameleer3/server/core/runtime/EnvironmentService.java

@@ -38,6 +38,14 @@ public class EnvironmentService {
         repo.updateDefaultContainerConfig(id, defaultContainerConfig);
     }
 
+    public void updateJarRetentionCount(UUID id, Integer jarRetentionCount) {
+        getById(id); // verify exists
+        if (jarRetentionCount != null && jarRetentionCount < 1) {
+            throw new IllegalArgumentException("Retention count must be at least 1 or null for unlimited");
+        }
+        repo.updateJarRetentionCount(id, jarRetentionCount);
+    }
+
     public void delete(UUID id) {
         Environment env = getById(id);
         if ("default".equals(env.slug())) {