env(admin): per-environment color field + V2 migration

- V2__add_environment_color.sql adds a CHECK-constrained VARCHAR color column (default 'slate'); existing rows backfill to slate.
- Environment record + EnvironmentColor constants (8 preset values) flow through repository, service, and admin API.
- UpdateEnvironmentRequest.color nullable: null preserves existing; unknown values → 400.
- ITs cover valid / invalid / null-preserves behaviour; existing Environment constructor call-sites updated with the new color arg.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -12,5 +12,6 @@ public record Environment(
         boolean enabled,
         Map<String, Object> defaultContainerConfig,
         Integer jarRetentionCount,
+        String color,
         Instant createdAt
 ) {}

cameleer-server-core/src/main/java/com/cameleer/server/core/runtime/EnvironmentColor.java

@@ -0,0 +1,28 @@
+package com.cameleer.server.core.runtime;
+
+import java.util.Set;
+
+/**
+ * Preset palette for the per-environment UI color indicator. Stored as a plain
+ * lowercase string on {@link Environment#color()}. The eight values are
+ * CHECK-constrained in PostgreSQL (V2 migration) and validated again here on
+ * the write path so the controller can return a 400 with a readable message.
+ *
+ * <p>Unknown values are silently tolerated on read (the UI falls back to
+ * {@link #DEFAULT}), so a manual DB tweak won't break rendering — but the API
+ * refuses to persist anything outside this set.
+ */
+public final class EnvironmentColor {
+
+    public static final String DEFAULT = "slate";
+
+    public static final Set<String> VALUES = Set.of(
+            "slate", "red", "amber", "green", "teal", "blue", "purple", "pink"
+    );
+
+    private EnvironmentColor() {}
+
+    public static boolean isValid(String color) {
+        return color != null && VALUES.contains(color);
+    }
+}

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

@@ -10,7 +10,7 @@ public interface EnvironmentRepository {
     Optional<Environment> findById(UUID id);
     Optional<Environment> findBySlug(String slug);
     UUID create(String slug, String displayName, boolean production);
-    void update(UUID id, String displayName, boolean production, boolean enabled);
+    void update(UUID id, String displayName, boolean production, boolean enabled, String color);
     void updateDefaultContainerConfig(UUID id, Map<String, Object> defaultContainerConfig);
     void updateJarRetentionCount(UUID id, Integer jarRetentionCount);
     void delete(UUID id);

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

@@ -43,9 +43,17 @@ public class EnvironmentService {
         return repo.create(slug, displayName, production);
     }
 
-    public void update(UUID id, String displayName, boolean production, boolean enabled) {
+    /**
+     * Update mutable environment fields. Color is validated against
+     * {@link EnvironmentColor#VALUES}. Unknown colors raise
+     * {@link IllegalArgumentException}; the controller maps that to 400.
+     */
+    public void update(UUID id, String displayName, boolean production, boolean enabled, String color) {
         getById(id); // verify exists
-        repo.update(id, displayName, production, enabled);
+        if (!EnvironmentColor.isValid(color)) {
+            throw new IllegalArgumentException("unknown environment color: " + color);
+        }
+        repo.update(id, displayName, production, enabled, color);
     }
 
     public void updateDefaultContainerConfig(UUID id, Map<String, Object> defaultContainerConfig) {