feat(groups): add fail-closed admission management

design/us-009-velocity-admission.md

@@ -3,7 +3,7 @@ type: User Story
 title: Enforce registration at the Velocity proxy
 description: Online-mode Java connections are admitted only after a fail-closed account-manager decision.
 tags: [minecraft, velocity, whitelist, security]
-timestamp: 2026-08-01T18:52:20Z
+timestamp: 2026-08-01T22:34:31Z
 story_id: US-009
 status: verified
 ---
@@ -22,7 +22,8 @@ As a registered player, I want the Velocity proxy to recognize my approved Java
 - [x] Username fallback applies only when the stored account has no UUID.
 - [x] Successful fallback backfills UUID and canonical username.
 - [x] Changed usernames are persisted and audited.
-- [x] Unknown players, API failures, malformed responses, and unauthorized requests fail closed with registration guidance.
+- [x] Registered players are allowed only when at least one assigned group has access enabled.
+- [x] Unknown players, group-disabled players, API failures, malformed responses, and unauthorized requests fail closed with registration guidance.
 - [x] The plugin records the real Velocity connection IP and supports Java Edition online mode only.
 
 # Implementation
@@ -41,3 +42,4 @@ As a registered player, I want the Velocity proxy to recognize my approved Java
 
 - [Validate Minecraft accounts](us-004-minecraft-validation.md)
 - [Standardize API errors](us-014-problem-details.md)
+- [Control Minecraft admission with groups](us-017-group-access.md)