Feat v0.6.15 user display audit

Add batch user resolve endpoint, sw.users SDK module, and migrate all
surfaces to show display_name instead of UUIDs. Canonical fallback chain:
display_name → username → "Unknown".

- GET /api/v1/users/resolve?ids=... (max 100, map response)
- sw.users.resolve(), resolveMany(), displayName() with 60s cache
- Chat participants resolved from users table, not snapshot
- Admin users/teams/groups/team-admin show display_name
- Chat-core participants.display_name column deprecated
- 5 new handler tests, OpenAPI spec updated

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

server/main.go

@@ -474,8 +474,9 @@ func main() {
 			protected.POST("/presence/heartbeat", presence.Heartbeat)
 			protected.GET("/presence", presence.Query)
 
-			// User search
+			// User search & resolve
 			protected.GET("/users/search", presence.SearchUsers)
+			protected.GET("/users/resolve", presence.ResolveUsers)
 
 			// Workflows
 			wfH := handlers.NewWorkflowHandler(stores)