Deprecate /picture/thumbnail endpoint

[Arechii]

Summary

• Mark /picture/thumbnail as deprecated in the HTTP API spec (deprecated: true + description)

Test plan

• Verify the endpoint renders as deprecated in the generated docs

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.50%. Comparing base (27d5e86) to head (6949af2).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #212   +/-   ##
=======================================
  Coverage   76.50%   76.50%           
=======================================
  Files          11       11           
  Lines        1664     1664           
=======================================
  Hits         1273     1273           
  Misses        391      391           

Flag	Coverage Δ
macos-latest_3.10	76.50% <ø> (ø)
macos-latest_3.11	76.50% <ø> (ø)
macos-latest_3.12	76.50% <ø> (ø)
macos-latest_3.13	76.50% <ø> (ø)
ubuntu-latest_3.10	76.50% <ø> (ø)
ubuntu-latest_3.11	76.50% <ø> (ø)
ubuntu-latest_3.12	76.50% <ø> (ø)
ubuntu-latest_3.13	76.50% <ø> (ø)
windows-latest_3.10	76.50% <ø> (ø)
windows-latest_3.11	76.50% <ø> (ø)
windows-latest_3.12	76.50% <ø> (ø)
windows-latest_3.13	76.50% <ø> (ø)

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[Arechii]