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/ent/runtime/runtime.go

@@ -270,6 +270,10 @@ func init() {
 	groupDescDefaultValidityDays := groupFields[10].Descriptor()
 	// group.DefaultDefaultValidityDays holds the default value on creation for the default_validity_days field.
 	group.DefaultDefaultValidityDays = groupDescDefaultValidityDays.Default.(int)
+	// groupDescClaudeCodeOnly is the schema descriptor for claude_code_only field.
+	groupDescClaudeCodeOnly := groupFields[14].Descriptor()
+	// group.DefaultClaudeCodeOnly holds the default value on creation for the claude_code_only field.
+	group.DefaultClaudeCodeOnly = groupDescClaudeCodeOnly.Default.(bool)
 	proxyMixin := schema.Proxy{}.Mixin()
 	proxyMixinHooks1 := proxyMixin[1].Hooks()
 	proxy.Hooks[0] = proxyMixinHooks1[0]