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

## 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

Start by locating the existing chart job and its upgrade hook; the hook slot, ordering, and privileged credentials are already in place. Trace the current command and the handling of installations that predate Alembic. Done means the chart runs the schema migration automatically when needed and documents the behavior.

Written by the indexing model from the issue text.