Fix runtime errors: evict_idle arg, writable temp dir

[mrSamDev]

Two runtime crashes in the container:

1. evict_idle() missing 1 required positional argument: idle_seconds. The maintenance loop called the bound method with no args, but it requires the idle threshold. It now passes SESSION_LEASE_TTL_SECONDS.

2. PermissionError: [Errno 13] Permission denied: temp. When TEMP_DIR is not applied at runtime the app falls back to /app/temp, which is root-owned and not created in the image. The image now pre-creates /app/temp owned by the non-root user, and main() resolves and creates temp/data/backups at startup, failing with a clear message instead of an opaque mid-request crash.