feat(sentinel): show field-level diff for modified certificates

[thiagoesteves]

Summary

• The certificate diff only reported that a certificate was modified, without showing which sub-fields changed. When an operator changed acme_options or dns_options, the config changes modal showed "Modified: DOMAINS" with the domain list, but no indication of what actually changed.
• Add a changes map to the modified certificate diff entry, listing each changed field (domains, acme_options, dns_options, importer_options, etc.) with its old and new values.
• Update the config changes modal to render each changed field with old -> new, following the same pattern used for monitoring changes.

Test plan

• mix test apps/sentinel/test/config/watcher_test.exs --warnings-as-errors (52 tests)
• mix test apps/deployex_web/test/deployex_web/live/applications/ --warnings-as-errors (76 tests)
• mix format --check-formatted
• mix credo --strict apps/sentinel/lib/sentinel/config/watcher.ex apps/deployex_web/lib/deployex_web/live/components/config_changes_modal.ex

Risk assessment

• Impact: Operators can now see exactly which certificate sub-fields changed in the config changes modal, instead of just "Modified".
• Blast radius: Sentinel config watcher diff_certificates and the web config_changes_modal certificate rendering; no apply logic changes.
• Regression risk: Low - the changes map is additive; existing consumers that only read status and config are unaffected.
• Rollback: Plain commit revert.

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