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/schemas/psa_connection.py

@@ -111,13 +111,13 @@ class PsaPostLogResponse(BaseModel):
 
 
 class PsaMemberMappingResponse(BaseModel):
-    id: str
+    id: str | None = None  # None for users without a mapping
     user_id: str
     user_email: str
     user_name: str
-    external_member_id: str
-    external_member_name: str
-    matched_by: str
+    external_member_id: str | None = None
+    external_member_name: str | None = None
+    matched_by: str | None = None
 
 
 class PsaMemberMappingSaveRequest(BaseModel):