fix(gateway): keep configured inverters when the hub cannot read them (#5471)

An inverter the hub could not read when Predbat started was left out of
auto-config and stayed out until the next restart, because every serial in
the status message was recorded as configured and so never counted as new.

With gateway_inverter_serial set, the list now decides the site:
- what the hub reports each listed inverter to be is kept in a discovery
  record and saved through the Storage component, updated only while the
  hub is reading that inverter;
- a listed inverter with a record keeps its slot when the hub cannot read
  it, including across a restart; one never read yet is left out with a
  warning until it is;
- records for serials no longer in the list are not used;
- a listed inverter with no battery is configured as PV only: no slot, no
  writes, its generation added to pv_power and pv_today;
- a configured inverter the hub is not reading logs an error and marks the
  run as having had errors until it returns.

With no serial list nothing is stored and selection is as before, except
that an inverter unusable at start-up is now registered when it becomes
usable.

An inverter with no readings keeps the entity values it last reported
instead of being published as zero.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: CI