*: feerecipient auto fetching

[pinebit]

Add automatic background fetching of builder registrations from the Obol API to the builderRegistrationService. Previously operators had to manually run charon feerecipient fetch to pull fully signed registrations. Now the service periodically fetches from the API and merges results with file overrides, preferring the entry with the highest timestamp per validator.

Fetching schedule:

• Immediate fetch on charon start
• Every 1h if any validators have incomplete (partially signed) registrations
• Every 24h if all registrations are fully signed
• File override changes also trigger an immediate API fetch

The Run function now works even when no overrides file path is configured, as long as the Obol API address is set.

category: feature
ticket: none

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 23.27044% with 122 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.26%. Comparing base (c381f32) to head (19caf7c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
app/builderregistration.go	22.44%	106 Missing and 8 partials ⚠️
app/app.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4407      +/-   ##
==========================================
+ Coverage   56.21%   56.26%   +0.04%     
==========================================
  Files         242      244       +2     
  Lines       32207    32508     +301     
==========================================
+ Hits        18106    18290     +184     
- Misses      11780    11875      +95     
- Partials     2321     2343      +22     

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