[DPE-9010] fix(backups): reject standby backup actions and harden async replication

[marceloneppel]

Issue

In async replication setups, backup actions on standby clusters should fail with clear guidance to run on the primary cluster. Today those actions can still run and fail later with less actionable errors.

Also, create-replication could hit an uncaught StopIteration when the relation exists but no remote units have published addresses yet.

Solution

• Add standby-cluster detection in PostgreSQLBackups and fail early for create-backup, list-backups, and restore with explicit action-specific messages.
• Harden async replication relation handling by failing early when the relation is missing or has no remote units yet, and by guarding relation-changed re-emission when there are no related units.
• Add a Juju3/Ceph-backed integration test that verifies backup works on the primary cluster and is rejected on the standby cluster with the expected message.
• Update the microceph fixture to use a non-loopback host IP for TLS SAN/certificate usage (this allows local testing in more environments).

Checklist

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

Fixes #1329.

[codecov]

Codecov Report

❌ Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.59%. Comparing base (9af522d) to head (87581fe).
⚠️ Report is 2 commits behind head on 16/edge.

Files with missing lines	Patch %	Lines
src/relations/async_replication.py	66.66%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge    #1602      +/-   ##
===========================================
+ Coverage    70.43%   70.59%   +0.16%     
===========================================
  Files           15       15              
  Lines         4282     4309      +27     
  Branches       694      700       +6     
===========================================
+ Hits          3016     3042      +26     
+ Misses        1057     1056       -1     
- Partials       209      211       +2     

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[taurus-forever]

THANK YOU! ❤️