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>

backend/app/services/psa/connectwise/provider.py

@@ -536,7 +536,7 @@ class ConnectWiseProvider(PSAProvider):
         if work_type:
             payload["workType"] = {"name": work_type}
 
-        data = await self._client.post("/time/entries", payload)
+        data = await self.client.post("/time/entries", payload)
         return PSATimeEntry(
             id=str(data["id"]),
             ticket_id=ticket_id,