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>

.claude/rules/core-classes.md

@@ -26,7 +26,8 @@ paths:
 
 - `App` — record: id, environmentId, slug, displayName, containerConfig (JSONB)
 - `AppVersion` — record: id, appId, version, jarPath, detectedRuntimeType, detectedMainClass
-- `Environment` — record: id, slug, jarRetentionCount
+- `Environment` — record: id, slug, displayName, production, enabled, defaultContainerConfig, jarRetentionCount, color, createdAt. `color` is one of the 8 preset palette values validated by `EnvironmentColor.VALUES` and CHECK-constrained in PostgreSQL (V2 migration).
+- `EnvironmentColor` — constants: `DEFAULT = "slate"`, `VALUES = {slate,red,amber,green,teal,blue,purple,pink}`, `isValid(String)`.
 - `Deployment` — record: id, appId, appVersionId, environmentId, status, targetState, deploymentStrategy, replicaStates (JSONB), deployStage, containerId, containerName
 - `DeploymentStatus` — enum: STOPPED, STARTING, RUNNING, DEGRADED, STOPPING, FAILED
 - `DeployStage` — enum: PRE_FLIGHT, PULL_IMAGE, CREATE_NETWORK, START_REPLICAS, HEALTH_CHECK, SWAP_TRAFFIC, COMPLETE