Throw conflict on authorising new Garmin user if external_user_id already exists

[yatharthranjan]

This should be unique and should throw a conflict exception. This is because Garmin will PUSH the data and our system wouldn't know which ID to map it to if more than one user is authorised on same external garmin user ID.

Example database entries

 id  | project_id |               user_id                |              source_id               | source_type |     start_date      |      end_date       |      
     external_user_id           | authorized |             access_token             |            refresh_token            | expires_in |         expires_at      
   | token_type |            version             | times_reset |         created_at         
-----+------------+--------------------------------------+--------------------------------------+-------------+---------------------+---------------------+------
--------------------------------+------------+--------------------------------------+-------------------------------------+------------+-------------------------
---+------------+--------------------------------+-------------+----------------------------
 285 | NSHD       | 6b1a5591-e0e5-45e1-9ea0-56673f86ea59 | f2f8d1ac-3981-4818-a445-079e59f25409 | Garmin      | 2023-10-23 23:00:00 | 2025-10-23 23:00:00 | 86e2d
28b-d362-4c41-8f54-d607800ea9ad | t          | 1a8*****************ad3452 |*********** | 2147483647 | 2091-11-11 18:10:00.2177
68 |            | 2023-10-24T15:00:43.478475883Z |           0 | 2023-10-24 15:00:43.478476
 777 | NSHD       | efb5a8bc-1db1-4e14-a2bb-e527cf4bb83b | f98e17b0-8d9a-462d-8715-7423a3628ab8 | Garmin      | 2024-09-08 23:00:00 | 2025-07-30 23:00:00 | 86e2d
28b-d362-4c41-8f54-d607800ea9ad | t          | 346************cd045 | ******** | 2147483647 | 2092-09-27 18:26:14.6147
44 |            | 2024-09-09T15:17:01.718895813Z |           0 | 2024-09-09 15:17:01.718896

[yatharthranjan]

I think this should be same for all the source_types, when authorised=true already exists with the external_user_id on the same source_type then it should throw an error, instead of linking the vendor account to a different user again.