docs: clarify PostgreSQL is only required for auth_mode=session

[bhavyamsharmaa]

Problem

Both README.md and docs/GETTING_STARTED.md list PostgreSQL as a hard prerequisite upfront. This causes new contributors to spend time setting up a database when it is not always needed.

The backend skips the database connection entirely when auth_mode is none or header (lib/db.go:17):

if env.AuthMode == "header" || env.AuthMode == "none" || !env.DBEnabled {
    logger.Info("Skipping database connection (auth mode is " + env.AuthMode + ")")
    return Database{DB: nil}
}

Proposed fix

Add a note near the PostgreSQL prerequisite in both files:

PostgreSQL is only required when using auth_mode: session (the default). For quick local development, set auth_mode: none in config/config.yaml to skip the database entirely.

config/examples/config-none.yaml.example already provides a ready-made config for this.

Impact

Reduces setup friction for new contributors and makes the "no-DB" path discoverable without reading the full configuration guide.