Add key holders list endpoint so creators can see who holds their keys

[Chucks1093]

Summary

Creators currently have no way to see who is in their community. Knowing which wallets hold your keys — and how many — is basic information a creator needs to understand their audience and reward their most loyal supporters. This endpoint turns the key ownership read model into a useful creator-facing feature.

Scope

• Add GET /creators/:id/holders endpoint
• Response returns a paginated list of: { wallet_address, key_balance, held_since }
• held_since is the timestamp of the wallet's first buy for this creator
• Default sort is by key_balance descending (largest holders first)
• Support sort=held_since to surface earliest supporters first
• Entries are sourced from the indexer's ownership read model

Acceptance Criteria

• Returns paginated list of holder wallets with correct balances
• Default sort is largest balance first
• sort=held_since returns earliest buyers first
• held_since reflects the timestamp of the wallet's first buy transaction for that creator
• Returns 404 if the creator does not exist
• Returns empty list (not 404) if creator exists but has no holders
• Integration test covers: multiple holders, empty holders, both sort modes

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Coordinate on Telegram

[shadrach68]

I would like to work on this issue because I am a full-stack developer with 5+ years experience. I focus on identifying root causes and delivering clean, efficient, and maintainable solutions, with thorough testing to ensure reliable results.