[Feature]: Run schema migrations from the chart (replace init-sql create_all)

[aldbr]

User Story

As a DiracX administrator,
I want helm upgrade to bring my databases to the schema the new release expects,
So that upgrading DiracX does not require me to find and run ALTER TABLE statements from pull-request
descriptions or Wiki pages.

Feature Description

Once Alembic lands in diracx-db, the chart has to drive it. The good news, and
the explicit scope agreed in discussion: this is one simple job that runs at the beginning of the
upgrade, and nothing more. The hook slot, the ordering and the privileged credentials already exist and
are already correct. Only the command inside the job changes, plus the handling of installations that
predate Alembic.

Definition of Done

Chart

• Schema Migration is performed automatically when needed
• Documentation

Alternatives Considered

Additional Context