feat(charms): complete the VM charm class (7/7)

[marceloneppel]

Issue

The VM charm class must become concrete in the library so the charm-side file reduces to a thin subclass (final state: no logic charm-side).

Solution

• PostgreSQLVMCharm: full composition bodies for every abstract seam — workload/substrate/postgresql/primary_endpoint properties, the refresh-aware status setters, the update_config wrapper (per-relation inputs + DPE-10203 stale-standby clearing), post-refresh side effects, exporter/ldap snap glue, CA push/clean, TLS reload/regenerate bridges, patroni_scrape_config, the disable-file guard, and the observer/rotate-logs init starts.
• utils/tracing.py: the shared charm_tracing_config helper (ported verbatim; both charms call it).
• The K8s charm class's update_config signature widened to the shared seam.
• Tests: tests/unit/test_vm_charm.py (status-setter priority, DPE-10203 clearing, primary/degraded/read-only/blocked-by-s3 messages).
• The charm-side file reduces to a thin subclass + COS requirer wiring in the consumption PR.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.