One edition: the last enterprise gates come out of shared code (SPEC 2.3)

Every one of the 14 was `#[cfg(not(feature = "enterprise"))]` on the arm the
fork always compiles: the Enterprise arms went with the import, and nothing
turns the feature on. Removing the attribute leaves the same code, now
unconditional, in 11 files.

Two of them looked like behavior worth checking before touching: the
`validate_tenant_quota` stub that always passes, and the refusal to cancel a
pending DestroyAccount task. The stub is vestigial — the rebuilt
multi-tenancy enforces quotas in `crates/features/src/tenancy/quota.rs` for
those objects and more — and the refusal is undelete's open question, which
this change leaves exactly as it was.

The binary builds with no new warnings, and `system_tests` and `jmap_tests`,
which cover the touched registry, task-manager and auth paths, both pass.
The feature definitions stay in the manifests, inert: taking them out would
widen every sync's diff for nothing.

docs/spec/SPEC.md

@@ -178,7 +178,13 @@ checks already are: 30 checks across 18 files at the commit above. That keeps
 each sync's conflicts few and predictable.
 
 The `enterprise` Cargo feature and every edition check are removed. There is
-one edition.
+one edition. Done for the code on 2026-09-19: the last 14 gates, all of them
+`#[cfg(not(feature = "enterprise"))]` on the path the fork always takes, came
+off across 11 files, and no `is_enterprise_edition()` check survived the
+import. The feature's *definitions* stay in the manifests, inert now that
+nothing turns them on and nothing reads them, because removing them would
+widen every sync's diff for no gain (§2.2). An import that changes a gated
+file brings its gates back in the merge; they come off again with it.
 
 One exception: scale-out storage lives in new AGPL files inside
 `crates/store` (`backend/scaleout/`), because the rebuilt-features crate