feat(registry): add SN50 Synth public miner leaderboard API surface

[minion1227]

Summary

Adds one community subnet-api surface to registry/subnets/synth.json (SN50 Synth),
live-verified public + no-auth:

• subnet-api: https://synthdata.co/api/leaderboard
  Returns the live miner leaderboard (rank, miner_uid, coldkey, scores) as JSON. 200, no credentials.

Surface

• Netuid: 50
• Kind: subnet-api
• Public URL: https://synthdata.co/api/leaderboard
• Source URL: https://synthdata.co/ + https://synthdata.co/docs
• Provider slug: synth

Validation

• npm run validate:surface -- registry/subnets/synth.json ✓
• npm run scan:public-safety ✓

Closes #669

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.11%. Comparing base (7a7b0c3) to head (e651769).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1970   +/-   ##
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  Coverage   93.11%   93.11%           
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  Files          46       46           
  Lines        7319     7319           
  Branches     2694     2694           
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  Hits         6815     6815           
  Misses         90       90           
  Partials      414      414           

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[JSONbored]

Closing this, and I want to be transparent about why.

This account is permanently blocked from contributing to our repositories — for plagiarism. In JSONbored/gittensory you filed two PRs that were copies of another contributor's open work, submitted as your own to claim the credit:

• JSONbored/gittensory#1377 was an identical copy of kiannidev's JSONbored/gittensory#1376 (same source change; tests only reworded to disguise the lift), filed ~90 minutes later against the same issue.
• JSONbored/gittensory#1378 was a byte-identical copy of kiannidev's JSONbored/gittensory#1373, filed ~2 hours later against the same issue.

Lifting another contributor's diff to claim the credit is a hard line for us — two strikes, you're out. Our Code of Conduct states it plainly: plagiarism and reward-farming result in a permanent block from contributing, enforced across JSONbored/gittensory, JSONbored/metagraphed, and JSONbored/awesome-claude.

The block is on the account, not any individual PR, so this isn't a review of the change — it's enforcement of the ban. Closed and labeled slop. This decision is final.