Fix import of snapshot credentials for instances

[knolleary]

Description

The snapshot import was not re-encrypting the credentials for the target owner. This was okay for Devices as they reuse the credsec of the snapshot - but not for instances that use their own.

If importing for an instance, this will now reencrypt the credentials appropriately.

Tests updated to cover it.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.12%. Comparing base (9e000e5) to head (7d50876).
Report is 15 commits behind head on main.

Files	Patch %	Lines
forge/db/controllers/Snapshot.js	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4262      +/-   ##
==========================================
- Coverage   78.12%   78.12%   -0.01%     
==========================================
  Files         287      287              
  Lines       13284    13288       +4     
  Branches     2977     2979       +2     
==========================================
+ Hits        10378    10381       +3     
- Misses       2906     2907       +1     

Flag	Coverage Δ
backend	78.12% <80.00%> (-0.01%)	⬇️

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