feat(db): make statefulset serviceName and data subPath configurable

[dhcsousa]

What kind of change does this PR introduce?

Feature (opt-in, backward compatible).

What is the current behavior?

Closes #241. The db StatefulSet hardcodes two values: serviceName (always supabase.db.fullname) and the data mount's subPath (always postgres-data). Since a StatefulSet's serviceName is immutable, any existing install whose object carries a different (or empty) value fails helm upgrade with a "field is immutable" error, and volumes provisioned with data at a different location (e.g. the volume root) can't be mounted correctly.

What is the new behavior?

Adds two optional values, both unset by default so the default render is unchanged:

• deployment.db.serviceName: used verbatim when the key is present (including an explicit "", which some legacy StatefulSets need to reproduce in order to upgrade in place); falls back to supabase.db.fullname when absent.
• persistence.db.subPath: defaults to postgres-data; setting it to "" mounts the volume root.

Additional context

helm lint passes and the default helm template output is byte-identical to main.