fix: use tmpfs for postgres data in E2E compose to prevent Flyway checksum mismatches

The postgres:16 image declares a VOLUME for /var/lib/postgresql/data.
Docker Compose creates an anonymous volume that persists across CI runs.
When a Flyway migration file is modified, the next run sees a checksum
mismatch because the old migration is already recorded in the schema_history
table in the stale volume.

- Add tmpfs: [/var/lib/postgresql/data] to the postgres service
- This keeps data in RAM only, guaranteeing a completely fresh database
  on every E2E run with no persistent state between invocations

Result: eliminates FlywayValidateException caused by migration checksum
mismatches in CI.

docker-compose.e2e.yml

@@ -12,6 +12,8 @@ services:
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
     networks:
       - e2e
+    tmpfs:
+      - /var/lib/postgresql/data
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB}"]
       interval: 5s