feat(config): resolve ${file:...} and ${VAR} references in config

[matt-codecov]

GCP secrets are mounted as files. this is what i did to load JWT keys in Relay's config, and it's more ergonomic than what i did for JWT keys in our own config. this will be used to load Kafka auth params

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.53%. Comparing base (329169e) to head (3ab8e17).

Additional details and impacted files

@@                          Coverage Diff                          @@
##           matth/storage-inventory-tracker-4     #599      +/-   ##
=====================================================================
+ Coverage                              88.46%   88.53%   +0.07%     
=====================================================================
  Files                                    104      104              
  Lines                                  17162    17277     +115     
=====================================================================
+ Hits                                   15182    15297     +115     
  Misses                                  1980     1980              

Components	Coverage Δ
Rust Backend	92.55% <100.00%> (+0.06%)	⬆️
Rust Client	81.97% <ø> (ø)
Python Client	93.31% <ø> (ø)

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[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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^{Reviewed by Cursor Bugbot for commit 476aa6d. Configure here.}