Refactor persistence commands for DB migration

[pablonete]

Blob-backed read-modify-write paths are making the storage layer hard to migrate safely to Netlify DB. This PR introduces the first migration slice: a command-oriented persistence boundary that preserves current file/blob behavior while giving future DB and dual-write adapters explicit operations to implement.

Summary

• Adds store commands for kid registration, passport completion, prize catalog writes, prize awards, and admin restore.
• Rewires API write handlers to call those commands instead of generic snapshot mutators.
• Keeps app-owned IDs and timestamps explicit where dual-write correctness depends on deterministic values.
• Shares the command helpers across file and Netlify Blob adapters so current behavior stays aligned.
• Updates storage docs to point future writable state migration toward Netlify DB behind the store interface.

Notes for review

• This does not add the Netlify DB adapter yet; it prepares the seam for the next PR.
• Magic-link tokens remain on their existing token store in this slice, but are planned for the first DB migration scope.
• No UI changes; screenshots are N/A.

Validation

• npm run --silent lint
• npm run --silent build
• npm run --silent build:server

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