postgresql: Deploy Postgres Operator

While I was preparing to deploy PostgreSQL for Firefly III, I was
thinking it would be a neat idea to write an operator that uses
custom resources to manage PostgreSQL roles and databases.  Then I
though, surely something like that must exist already.  As it turns out,
the [Postgres Operator][0] does exactly that, and a whole lot more.

The *Postgres Operator* handles deploying PostgreSQL server instances,
including primary/standby replication with load balancers.  It uses
custom resources to manage the databases and users (roles) in an
instance, and stores role passwords in Secret resources.  It supports
backing up instances using `pg_basebackup` and WAL archives (i.e.
physical backups) via [WAL-E][1]/[WAL-G][2].  While various backup
storage targets are supported, *Postgres Operator* really only works
well with the cloud storage services like S3, Azure, and Google Cloud
Platform.  Fortunately, S3-compatible on-premises solutions like MinIO
are just fine.

I think for my use cases, a single PostgreSQL cluster with multiple
databases will be sufficient.  I know *Firefly III* will need a
PostgreSQL database, and I will likely want to migrate *Paperless-ngx*
to PostgreSQL eventually too.  Having a single instance will save on
memory resources, at the cost of per-application point-in-time recovery.
For now, just one server in the cluster is probably sufficient, but
luckily adding standby servers appears to be really easy should the need
arise.

[0]: https://postgres-operator.readthedocs.io/en/latest/
[1]: https://github.com/wal-e/wal-e
[2]: https://github.com/wal-g/wal-g

postgresql/.gitignore

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+pod.secrets