Add vesting claim, key burn, timelock, and supply cap endpoints

[luciachizaram]

Adds four new endpoints across vesting, keys, and admin modules.

• POST /vesting/:keyId/claim — claims vested keys, updates claimedAmount, writes activity log
• POST /keys/:keyId/burn — validates holder balance, decrements balance + circulatingSupply atomically
• POST /admin/timelock/propose, execute, cancel, GET proposals — timelock proposal management with 48h delay
• POST /creator/:keyId/supply-cap — validates cap >= circulatingSupply, updates supplyCap

Fixes #822,Fixes #823,Fixes #824,Fixes #825

[drips-wave]

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You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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