feat(deploy): auto-run db migrations + seed on stack deploy

Add a migrator Dockerfile target (drizzle-kit migrate + tier-definitions
seed, both idempotent) and wire it as a one-shot compose service that web
waits on via service_completed_successfully. No more manual exec step
after first deploy — runs safely on every redeploy.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Dockerfile

@@ -11,6 +11,12 @@ COPY packages/db/package.json packages/db/package.json
 COPY packages/api-types/package.json packages/api-types/package.json
 RUN pnpm install --frozen-lockfile
 
+# ---- migrator: applies drizzle migrations + seeds tier definitions ----
+FROM deps AS migrator
+WORKDIR /repo/packages/db
+COPY packages/db .
+CMD ["sh", "-c", "pnpm migrate && pnpm seed"]
+
 # ---- build ----
 FROM base AS build
 WORKDIR /repo