compose: pin MOSAIC_STORAGE_TIER=standalone in the stack profile

Formal A5d run on the merged branch surfaced a tier-detection probe
race: probePostgres (5s connect) failed against a HEALTHY postgres on
the same network (psql SELECT 1 from a peer container succeeds; the
wrapped cause is not surfaced by the bootstrap log). The compose is the
standalone tier BY DECLARATION (mode contract: mode chosen at install)
— pinning MOSAIC_STORAGE_TIER=standalone uses the documented env path
('storage tier=standalone source=process environment', observed in the
first scratch run) and removes the boot-order dependency on probe
timing entirely.

docker-compose.yml

@@ -64,6 +64,9 @@ services:
       GATEWAY_PORT: '14242'
       DATABASE_URL: postgresql://mosaic:mosaic@postgres:5432/mosaic
       VALKEY_URL: valkey://valkey:6379
+      # The compose IS the standalone tier by declaration (mode contract:
+      # mode chosen at install); pinning skips cross-container probe races.
+      MOSAIC_STORAGE_TIER: standalone
       # Standalone-tier secrets: generated at install (see .env.example).
       # Enterprise tier replaces these with Vault/Openbao plumbing.
       BETTER_AUTH_SECRET: '${BETTER_AUTH_SECRET:?set in .env — openssl rand -hex 32}'