The tenants.* methods were the only RPC group without MethodXxx
constants in pkg/protocol/methods.go. Their wire names were hardcoded
as raw string literals across three call sites — router registration,
and the read/admin allowlists in the permission policy — which is
inconsistent with the other 147 methods and risks silent drift: a
typo in any allowlist string is not caught by the compiler and would
fail-closed (deny) at runtime, which is hard to diagnose.
Add MethodTenants{List,Get,Create,Update,UsersList,UsersAdd,
UsersRemove,Mine} and replace the method-identifier literals in
internal/permissions/policy.go and internal/gateway/methods/tenants.go
with the constants.
No behavior change — string values are identical. Human-facing error
labels and log prefixes in internal/http/tenants.go are intentionally
left as free-form text, matching the sibling HTTP handler convention.
