refactor: 🏄🏽 move app logic into new module

[rodneylab]

Description

Moved app logic to its own module. This decouples the metrics and tracing globals from the router, and should help in writing cleaner and simpler tests.

Type of change

• Refactor

How Has This Been Tested?

• cargo test run with all tests passing

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 26.41509% with 78 lines in your changes missing coverage. Please review.

Project coverage is 86.82%. Comparing base (903636b) to head (d731474).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/startup.rs	24.73%	70 Missing ⚠️
src/main.rs	0.00%	7 Missing ⚠️
src/observability/metrics.rs	75.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   88.68%   86.82%   -1.86%     
==========================================
  Files           8        9       +1     
  Lines        1228     1230       +2     
==========================================
- Hits         1089     1068      -21     
- Misses        139      162      +23     

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