Since existing Blob and DB data is only dirty testing data, this PR skips old-data migration and moves the rollout toward a clean reset plus dual writes. It lets operators reset both stores from committed seed data, then run with Blob reads while shadow-writing Netlify DB.

Summary

• Adds dual store composition that reads from the configured primary backend and mirrors writes to the secondary backend.
• Adds dual magic-token storage so tokens follow the same backend mode as app data.
• Supports KID_A_STORE_BACKEND=dual, KID_A_STORE_READ=blob|db, and KID_A_DUAL_WRITE_STRICT.
• Removes the separate KID_A_TOKEN_BACKEND path by always deriving token storage from KID_A_STORE_BACKEND.
• Adds npm run data:reset-stores to apply DB schema, reset Blob and DB writable data from seed JSON, and clear magic-link tokens in both stores.
• Updates storage docs with the reset and dual-write rollout flow.

Notes for review

• Blob remains the default backend.
• The intended rollout is reset both stores, run dual writes with Blob reads, then switch reads to DB in a follow-up PR.
• 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