Clarify that --with-default-root-credentials only apply on first server start

[YangSiJun528]

Issue

The --with-default-root-credentials flag only works on first server start. (when no local_data directory exists)
On subsequent starts, the flag is ignored but the server still logs:

INFO iggy_server: Using default root credentials (username: iggy, password: iggy) - FOR DEVELOPMENT ONLY!

This misleads users into thinking the credentials are active when they're not.

This also affects users who manually set IGGY_ROOT_USERNAME and IGGY_ROOT_PASSWORD environment variables after the server has already created a root user.

Steps to Reproduce

# First run - creates root user with random password
cargo run --bin iggy-server
# Stop and restart with flag
cargo run --bin iggy-server -- --with-default-root-credentials
# Authentication fails
cargo run --example getting-started-producer

Root Cause

The flag sets IGGY_ROOT_USERNAME=iggy and IGGY_ROOT_PASSWORD=iggy environment variables, but load_users() in core/server/src/streaming/systems/users.rs only creates a new root user when users.is_empty().
Since local_data already contains the user from the first run, the flag is silently ignored.

Proposed Fix

• Add warning log when flag/environment variables are used but users already exist
• Add information in README that existing user data persists across restarts
• Update CLI help text to clarify this behavior (CLI help already mentions this)