Fixes wrong signer for custom contact files

[tompro]

📝 Description

Fixes invalid keyset for custom contact file uploads

Relates to #911

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✅ Checklist

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[Copilot]

Pull request overview

Fixes contact file uploads for external contacts by ensuring file-metadata publishing uses a node id derived from the available signer keys (instead of the contact’s node id), addressing the “No signer found for node_id: ” failure described in #911.

Changes:

• Renames the upload-processing parameter to clarify it represents the contact’s node id.
• When encrypting/saving an uploaded contact file, derives a publisher_node_id from signer.pub_key() and uses it for publishing file metadata events.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.71%. Comparing base (de6611f) to head (aa0c7da).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #914      +/-   ##
==========================================
+ Coverage   70.58%   70.71%   +0.13%     
==========================================
  Files         139      139              
  Lines       27708    27714       +6     
==========================================
+ Hits        19557    19599      +42     
+ Misses       8151     8115      -36     

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