Add Netlify DB store adapter

[pablonete]

This continues the storage migration away from eventually consistent blob writes by adding the first Netlify DB-backed persistence implementation. The blob backend remains the default, but Netlify can now opt into DB-backed data and magic-token stores through explicit config.

Summary

• Adds the initial Postgres schema for conference settings, kids, passports, prizes, prize awards, and magic-link tokens.
• Implements a Netlify DB store adapter using @netlify/neon behind the existing command-oriented store interface.
• Preserves app-owned IDs for kids, prizes, awards, and token hashes; the database does not generate user-facing IDs or maintain a separate kid counter table.
• Derives prize given counts from prize awards and keeps passport state in relational rows.
• Adds a DB-backed magic-link token store and Netlify function config switches for data and token backends.
• Documents the DB backend toggle and schema location.

Notes for review

• Blob storage remains the default runtime backend.
• Migration/population tooling and dual-write rollout are intentionally left for follow-up PRs.
• No UI changes; screenshots are N/A.

Validation

• npm run --silent lint
• npm run --silent build
• npm run --silent build:server
• git --no-pager diff --check

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