feat(security): generate random JWT secrets when not configured

[spetz]

Deploying the HTTP server with hardcoded default JWT secrets
is a security risk — every instance shares the same signing
key. Empty defaults now trigger secure random secret
generation at startup, with a warning logged showing a
redacted preview of the generated value.

When both encoding and decoding secrets are empty, a single
random secret is generated and used for both (symmetric
HMAC). Tokens are invalidated on restart, nudging operators
to set persistent secrets in production.

[codecov]

Codecov Report

❌ Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.80%. Comparing base (2119848) to head (d2db5f1).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/server/src/http/http_server.rs	55.00%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2974      +/-   ##
============================================
- Coverage     71.81%   71.80%   -0.02%     
  Complexity      930      930              
============================================
  Files          1116     1116              
  Lines         92601    92616      +15     
  Branches      70122    70149      +27     
============================================
- Hits          66500    66499       -1     
- Misses        23539    23551      +12     
- Partials       2562     2566       +4     

Flag	Coverage Δ
csharp	67.43% <ø> (-0.23%)	⬇️
go	36.38% <ø> (ø)
java	62.08% <ø> (ø)
node	91.37% <ø> (-0.17%)	⬇️
python	81.43% <ø> (ø)
rust	72.50% <55.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
core/server/src/http/http_server.rs	48.60% <55.00%> (+0.30%)	⬆️

... and 19 files with indirect coverage changes

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