V196: a user's Google address.

Adds google_email to phoenix_kit_users. Sharing something with a user through Google (a Drive file, a calendar invite) needs the address their Google account actually answers to, which is not always the address they registered with.

The column is nullable and unconstrained: a Google address is optional, several users may legitimately share one (a team inbox), and nothing authenticates against it — email stays the identity.

The up also backfills it from phoenix_kit_user_oauth_providers: a user who has linked a Google sign-in already proved that address to us, so their column starts filled rather than waiting for their next sign-in. Only rows whose provider_email is present are copied, and only where google_email is still NULL, so the backfill is re-runnable and never overwrites a hand-entered value. The address is stored the way the changeset stores it — trimmed and lower-cased — and one longer than the column is skipped: provider_email is varchar(255) and this column is varchar(160), so copying it verbatim would raise "value too long" and strand the host below V196 over a single row. A user with more than one Google link (the table allows one row per provider per user, but a repaired install may carry duplicates) takes the most recently updated one.

Additive only; re-runnable.