fix(garm): make application removal safe during relation teardown

## Bug description

Removing the GARM application can race with Juju relation teardown. Relation-departed/broken hooks re-enter the normal reconcile path after relation data has already disappeared, causing the removal workflow to fail before or during application teardown.

## Evidence from production removal test

During a targeted removal of `github-runner-garm`:

- `ingress-relation-departed` failed with the relation snapshot warning `"app" expected but not received`.
- `garm-configurator-relation-broken` failed repeatedly with the same snapshot warning and `secret_get` errors while reconstructing charm state.
- `postgresql-relation-broken` and `debug-ssh-relation-broken` also observed missing relation snapshot data.
- The unit remained in error/awaiting error resolution until failed hooks were marked resolved without retry.
- A remaining cross-model debug-ssh relation had to be force-removed before the dying application disappeared.

Relevant Juju bug reference emitted by the relation libraries: https://bugs.launchpad.net/juju/+bug/1960934

## Expected behavior

The remove path should not run normal relation-driven reconciliation after teardown has started. Relation-departed/broken handlers should tolerate missing snapshot data during application removal, and the dedicated `on.remove` cleanup should run while GARM and its database/API are still available.

## Suggested direction

- Gate or short-circuit relation handlers when the application is dying/removing.
- Ensure cleanup ordering does not depend on relation data that Juju may remove first.
- Add an integration test for application removal with all current relations, including cross-model relations.
- Preserve the GARM encryption secret or otherwise define database/secret lifecycle for application recreation.

## Environment

- PS7 production Kubernetes model: `k8s-pfe-ps7-prod-github-runner-prod`
- GARM revision: 104
- Test ticket: ISD-6082

Start by tracing the relation-departed and relation-broken handlers alongside the dedicated on.remove cleanup path, then inspect how application-dying state and relation snapshots are handled. Add the requested integration test covering removal with current and cross-model relations, and verify that hooks tolerate missing data while cleanup completes and the application is removed.

Written by the indexing model from the issue text.