file_persistence: true breaks standalone mode — "No catalog + schema named memory found"

When enabling file_persistence: true in standalone mode, every connection fails immediately after authentication
with:

Catalog Error: SET schema: No catalog + schema named "memory" found.

The file-backed DuckDB database is opened successfully (Opening file-backed DuckDB. path="/data/duckgres/test.duckdb?allow_unsigned_extensions=true"), extensions load, but the session setup
crashes when trying to switch to the memory catalog.

Steps to Reproduce

1. Configure duckgres.yaml:

host: "0.0.0.0"
port: 5432
data_dir: "/data/duckgres"
file_persistence: true

tls:
  cert: "/certs/server.crt"
  key: "/certs/server.key"

extensions:
  - parquet
  - httpfs

users:
  test: "test"

2. Run duckgres in standalone mode (default)
3. Connect with psql:
   psql "host=localhost user=test dbname=test sslmode=require"

Test Setup

• Image: ghcr.io/posthog/duckgres:latest
• Version: build-c6f9f04756b68cd4100b65374f2dff1b0f37657c
• Mode: standalone (default)
• DuckDB extensions: parquet, httpfs
• Duckgres is running in a swarm cluster

Start by reproducing the standalone configuration with file_persistence enabled and tracing the session setup that switches to the memory schema after authentication. Confirm the fix by connecting with psql against the file-backed database and verifying that sessions no longer fail with the catalog/schema error.