Integrate auth middleware into app builder and router

[jhrozek]

Wire authentication middleware into the application:

• Add auth middleware creation in app builder
• Mount RFC 9728 endpoint at /.well-known/oauth-protected-resource
• Create selective auth middleware for protected paths
• Define default public paths (health, docs, swagger, .well-known)
• Update builder tests for auth components

Fixes: #54

[codecov-commenter]

Codecov Report

❌ Patch coverage is 62.50000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.53%. Comparing base (201113d) to head (ebbc9d0).

Files with missing lines	Patch %	Lines
internal/api/server.go	0.00%	5 Missing ⚠️
internal/app/builder.go	63.63%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
- Coverage   66.56%   66.53%   -0.04%     
==========================================
  Files          60       60              
  Lines        3742     3765      +23     
==========================================
+ Hits         2491     2505      +14     
- Misses       1096     1103       +7     
- Partials      155      157       +2     

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