feat(groups): add Claude Code client restriction and session isolation

- Add claude_code_only field to restrict groups to Claude Code clients only
- Add fallback_group_id for non-Claude Code requests to use alternate group
- Implement ClaudeCodeValidator for User-Agent detection
- Add group-level session binding isolation (groupID in Redis key)
- Prevent cross-group sticky session pollution
- Update frontend with Claude Code restriction controls

backend/internal/repository/api_key_repo.go

@@ -325,6 +325,8 @@ func groupEntityToService(g *dbent.Group) *service.Group {
 		ImagePrice2K:        g.ImagePrice2k,
 		ImagePrice4K:        g.ImagePrice4k,
 		DefaultValidityDays: g.DefaultValidityDays,
+		ClaudeCodeOnly:      g.ClaudeCodeOnly,
+		FallbackGroupID:     g.FallbackGroupID,
 		CreatedAt:           g.CreatedAt,
 		UpdatedAt:           g.UpdatedAt,
 	}