Add a batch key metadata endpoint to fetch details for multiple key IDs in a single request

[Chucks1093]

Summary

The batch buy feature on the client requires metadata for multiple keys simultaneously. Fetching them one by one causes a waterfall of requests. A POST /keys/batch endpoint accepting an array of key IDs and returning their metadata in a single response would eliminate this.

Scope

• Add POST /keys/batch endpoint accepting { ids: string[] } in the request body
• Validate that the array contains between 1 and 20 key IDs
• Return an array of key metadata objects in the same order as the input IDs
• For IDs that do not exist return null in the corresponding position
• Cache individual key metadata in Redis for 60 seconds and serve from cache when available

Acceptance Criteria

• Returns metadata for all valid IDs in input order
• Unknown IDs return null in the corresponding position
• More than 20 IDs returns 400
• Empty array returns 400
• Cached metadata served without a database query

ETA: 12 hours

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Coordinate on Telegram

[Olowodarey]

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[Akpamgbo]

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