docs: clarify proxy threat model

[mcollina]

This relates to...

N/A

Rationale

Clarify the security threat model for proxy use and internal network deployments.

Changes

• State that internal/private network environments are trusted for traffic privacy.
• Clarify that configured proxies are trusted, authorized intermediaries for routing traffic, such as through a firewall.
• Document that anonymity, traffic hiding, and bypassing organizational, regulatory, or legal controls are non-goals.
• Add unauthorized or untrusted proxy usage as a non-vulnerability/deployment responsibility.

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

Tested with git diff --check -- SECURITY.md; commit hook also ran npm run lint.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (838b6cd) to head (7ee41b6).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5530      +/-   ##
==========================================
+ Coverage   93.43%   93.44%   +0.01%     
==========================================
  Files         110      110              
  Lines       37278    37329      +51     
==========================================
+ Hits        34829    34883      +54     
+ Misses       2449     2446       -3     

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