metadata.yaml doesn't set a limit on database relations

### Bug Description

it's possible to relate multiple databases to the databases endpoint, leading to `hook failed: storage-attached`

### To Reproduce

deploy with mysql-router and mysql and relate both to the database endpoint

### Environment

https://test-observer.canonical.com/#/charms/275993?testExecutionId=287586&testResultId=7944781

### Relevant log output

```shell
2025-11-18T16:04:19.264Z [container-agent] 2025-11-18 16:04:19 DEBUG jujuc server.go:302 running hook tool "juju-log" for target/0-uploads-storage-attached-7ce6e585ee5de350da0010ccd0fe6552
2025-11-18T16:04:19.264Z [container-agent] 2025-11-18 16:04:19 ERROR juju-log Uncaught exception while in charm code:
2025-11-18T16:04:19.264Z [container-agent] Traceback (most recent call last):
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/./src/charm.py", line 1580, in
2025-11-18T16:04:19.264Z [container-agent] main(WordpressCharm)
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/main.py", line 39, in main
2025-11-18T16:04:19.264Z [container-agent] return _main.main(charm_class=charm_class, use_juju_for_storage=use_juju_for_storage)
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/_main.py", line 504, in main
2025-11-18T16:04:19.264Z [container-agent] manager.run()
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/_main.py", line 488, in run
2025-11-18T16:04:19.264Z [container-agent] self._emit()
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/_main.py", line 423, in _emit
2025-11-18T16:04:19.264Z [container-agent] self._emit_charm_event(self.dispatcher.event_name)
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/_main.py", line 467, in _emit_charm_event
2025-11-18T16:04:19.264Z [container-agent] event_to_emit.emit(*args, **kwargs)
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/framework.py", line 351, in emit
2025-11-18T16:04:19.264Z [container-agent] framework._emit(event)
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/framework.py", line 924, in _emit
2025-11-18T16:04:19.264Z [container-agent] self._reemit(event_path)
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/framework.py", line 1036, in _reemit
2025-11-18T16:04:19.264Z [container-agent] custom_handler(event)
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/./src/charm.py", line 1565, in _reconciliation
2025-11-18T16:04:19.264Z [container-agent] self._core_reconciliation()
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/./src/charm.py", line 924, in _core_reconciliation
2025-11-18T16:04:19.264Z [container-agent] if not self._current_effective_db_info:
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/./src/charm.py", line 675, in _current_effective_db_info
2025-11-18T16:04:19.264Z [container-agent] relation = self.model.get_relation(self._DATABASE_RELATION_NAME)
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/model.py", line 277, in get_relation
2025-11-18T16:04:19.264Z [container-agent] return self.relations._get_unique(relation_name, relation_id)
2025-11-18T16:04:19.264Z [container-agent] File "/var/lib/juju/agents/unit-target-0/charm/venv/ops/model.py", line 1025, in _get_unique
2025-11-18T16:04:19.264Z [container-agent] raise TooManyRelatedAppsError(relation_name, num_related, 1)
2025-11-18T16:04:19.264Z [container-agent] ops.model.TooManyRelatedAppsError: Too many remote applications on database (2 > 1)
```

### Additional context

Part of the charm qa testing effort

Inspect the database endpoint declaration in metadata.yaml and compare it with the reproduction using mysql-router and mysql. Deploy the charm with both relations and verify that the relation limit prevents the storage-attached hook failure caused by multiple remote applications.

Written by the indexing model from the issue text.