Add admin toggle to disable entity hooks (insert/update)

Adds a "Disable link extraction on every entity save" checkbox to the settings form, so bulk operations (batch imports, migrations) on content with many links can skip the synchronous per-save extraction without needing custom code.

Backed by state, not config: this is an operational, per-environment switch someone flips on for the duration of a specific run, not a deployable site setting -- storing it in config would mean it gets pulled into config export/import and could accidentally travel to another environment (e.g. exporting "disabled" from a migration box and deploying that to production).

Only insert/update are gated. Delete still runs its cleanup regardless, so removed entities don't leave orphaned tracked links behind indefinitely.

Closes #3581562