fix: clean up ACME certs when resources are deleted

[shreyaspapi]

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Description

Fixes #2382

What's wrong

When you delete a resource, Traefik keeps its certificate in acme.json and keeps renewing it forever. The deleteResource handler only removes the DB row — it never touches the ACME storage.

What this does

• After deleting a resource, checks if any other resource still uses the same domain
• If not, removes that domain's certificate from acme.json so Traefik stops renewing it
• Adds a configurable acme_json_path option (defaults to /app/config/letsencrypt/acme.json)
• The cleanup is best-effort — if it fails for any reason, the delete still succeeds

How to test?

1. Create a resource with a custom domain that gets a Let's Encrypt cert
2. Delete that resource
3. Verify the domain's cert is removed from acme.json
4. Verify that domains shared by other resources are NOT removed