fix(infra): close db-startup race in supersync e2e stack

pg_isready -U supersync without -d returned OK as soon as postgres
accepted connections to the default database, but during first-run
initdb the server briefly bounced while POSTGRES_DB was created.
supersync's prisma db push then race-failed with P1001.

- Healthcheck now runs psql -d supersync_db -c 'SELECT 1' so it only
  passes once the app's db is queryable.
- Dockerfile.test entrypoint retries prisma db push up to 15x before
  giving up — defense in depth if anything else ever races.

docker-compose.yaml

@@ -11,11 +11,22 @@ services:
       - '55432:5432'
     volumes:
       - db_data:/var/lib/postgresql/data
+    # Gate on supersync_db existing AND accepting queries from the app user.
+    # Plain `pg_isready -U supersync` returns OK as soon as postgres accepts
+    # connections to the default `postgres` database — but during first-run
+    # initdb the server briefly bounces while POSTGRES_DB is created, so
+    # supersync's `prisma db push` race-fails with P1001. Querying
+    # supersync_db directly closes that window.
     healthcheck:
-      test: ['CMD-SHELL', 'pg_isready -U supersync']
-      interval: 5s
+      test:
+        [
+          'CMD-SHELL',
+          'pg_isready -U supersync -d supersync_db && psql -U supersync -d supersync_db -c "SELECT 1" > /dev/null 2>&1',
+        ]
+      interval: 3s
       timeout: 5s
-      retries: 5
+      retries: 20
+      start_period: 10s
 
   # SuperSync server for E2E testing
   # Start with: docker compose up -d supersync

packages/super-sync-server/Dockerfile.test

@@ -50,5 +50,10 @@ ENV TEST_MODE_CONFIRM=yes-i-understand-the-risks
 ENV CORS_ORIGINS=*
 ENV JWT_SECRET=e2e-test-secret-minimum-32-chars-long-for-security
 
-# Push schema to DB and start server
-CMD sh -c "npx prisma db push && node dist/src/index.js"
+# Push schema to DB and start server.
+# Retry `prisma db push` for up to ~30s on transient connection failures
+# (postgres can briefly bounce during first-run initdb even after the db's
+# healthcheck passes). Without this, the container exits on first P1001
+# and depends on docker's restart policy to recover, which can't be relied
+# on across compose invocations.
+CMD sh -c "for i in $(seq 1 15); do npx prisma db push && break; echo \"prisma db push failed (attempt $i/15), retrying in 2s...\"; sleep 2; done && node dist/src/index.js"