Remove retired provider migration files

[xwchris]

What changed

• remove the retired Supabase schema, local CLI configuration, migration runbook, and snapshot conversion tooling
• remove one-time cutover endpoints, secrets, write-freeze switches, and migration-only tests
• keep active Apple service-bound authentication covered by a focused test
• update Cloudflare/D1 documentation, licensing boundaries, issue metadata, and production verification

Why

Bellwire now runs on Better Auth and two Cloudflare D1 databases. The deleted files and code existed only for the completed provider cutover and no longer participate in runtime, deployment, self-hosting, or rollback.

Impact

Current Better Auth, D1, APNs, Private-first, and Hosted behavior is unchanged. Historical tracked files remain recoverable from Git history.

Validation

• npm test — 23 files, 165 tests passed
• npm run lint
• npm run typecheck
• npm run build
• npm run build:production
• git diff --check