When config.json schema changes between versions, register a migration
function in src/config/migrations.js. On startup, loadSiteConfig() detects
the stored version, runs all migrations forward, and writes the result back.
Users never see the migration — it runs silently and the rest of the app
only ever sees the current schema.
Includes:
- v0 → v1: normalize dns from string to object
- v1 → v2: add dns.provider field (default 'technitium')
- Forward compat: configs from future versions left untouched
- Idempotent: re-running on already-migrated config is a no-op
- Safe: no user data is removed during migration
21 unit tests covering edge cases: null input, forward compat, corrupt
JSON, missing parent dirs, idempotency, full migration chain.
