Introduce the data model behind a Google Health provider that reads
Fitbit, Pixel Watch, and Fitbit Air data through a user's Google account.
- GoogleHealthConnection: 1:1 with the user, encrypted access/refresh
tokens, token expiry, external Google user id, sync high-water-mark,
and a needsReauth flag for the periodic re-consent that Google's
Testing-mode clients require.
- GoogleHealthOAuthState: the connect-flow nonce ledger with the PKCE
verifier.
- Two encrypted BYO-client columns on User for the operator's own Google
OAuth client id and secret.
- GOOGLE_HEALTH added to the measurement_source enum.
Migration 0224 mirrors the existing integration shape: a standalone
enum-add, guarded CREATE TABLE / ADD COLUMN, forward-only and idempotent.
