feat(relay): provision the Vitess database ahead of the Postgres cutover

[juliusmarminge]

Phase 1 of the relay Postgres → Vitess migration (see the stacked cutover PR for the full picture and runbook).

Adds RelayMysqlDatabase (t3coderelay-vitess, us-west, PS_20) to the prod stack with the checked-in migrations/mysql baseline. Deploying this creates the empty Vitess database and applies the schema via the alchemy migration runner — relay_migrations bookkeeping included — while the worker keeps running on Postgres. That lets AWS DMS replicate production data into the target before the cutover PR flips Hyperdrive.

No worker or runtime changes; Postgres remains the live database.

🤖 Generated with Claude Code

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Note

Medium Risk
Prod infra now provisions an additional live database and applies schema on deploy; misconfiguration could affect migration bookkeeping or DMS target readiness, but runtime traffic stays on Postgres until cutover.

Overview
Phase 1 of the relay Postgres → Vitess migration: prod deploys now also provision PlanetScale MySQL (t3coderelay-vitess, us-west, PS_20) when relayDatabaseMode is shared-database, alongside the existing Postgres database.

A new checked-in MySQL baseline in migrations/mysql creates all relay_* tables and indexes (mirroring Postgres) and is applied via relay_migrations on deploy. The worker and Hyperdrive still use Postgres; nothing reads or writes MySQL until the cutover PR.

Adds the Drizzle snapshot for the MySQL baseline migration metadata.

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Note

Provision Vitess MySQL database ahead of Postgres cutover for relay service

• Adds a Planetscale MySQL database resource (t3coderelay-vitess, us-west, PS_20, 2 replicas) in db.ts when relayDatabaseMode returns "shared-database".
• Adds a baseline MySQL migration in migration.sql that creates all relay_* tables, indexes, and schema matching the existing Postgres structure.
• The Postgres database continues to be provisioned and used; the MySQL resource is additive at this stage.
• Behavioral Change: shared-database deployments now provision an additional database resource with RemovalPolicy.retain().

^{Macroscope summarized 394dead.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR provisions new production MySQL/Vitess database infrastructure ahead of a Postgres-to-Vitess migration. Production infrastructure changes of this significance warrant human review, even when the code diff is small.

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