fix(psa): fix time entry AttributeError and show all users in member mapping

- Fix create_time_entry() using self._client instead of self.client
- GET /member-mappings now returns all active account users, not just mapped
  ones — allows manual assignment when auto-match by email doesn't work
- PsaMemberMappingResponse mapping fields are now Optional (id, external_member_id,
  external_member_name, matched_by) to represent unmapped users
- Frontend MemberMappingTab skips null external_member_id when building
  localMappings, and derives user list from all returned entries
- Add docs/connectwise-psa-testing-checklist.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/types/integrations.ts

@@ -108,13 +108,13 @@ export interface PsaMemberResponse {
 }
 
 export interface PsaMemberMappingResponse {
-  id: string
+  id: string | null
   user_id: string
   user_email: string
   user_name: string
-  external_member_id: string
-  external_member_name: string
-  matched_by: string
+  external_member_id: string | null
+  external_member_name: string | null
+  matched_by: string | null
 }
 
 export interface AutoMatchResult {