fix(locator): keep users online while active bindings remain

[dengxiayehu]

Problem

The SIP registrar removes a binding through several independent paths: an
explicit REGISTER with expires=0, a transport close (e.g. a browser that
closes its WebSocket without unregistering), the periodic expired-registration
sweep, and lookup-time cleanup in the DB-backed locator.

A single registered identity (username + realm) may hold multiple active
bindings at once — for example, when a client reconnects and registers a fresh
Contact before the previous Contact expires or its transport is torn down.
Removing any one binding used to publish LocatorEvent::Offline / Unregistered
for the removed bindings directly. A stale binding could therefore overwrite the
fresh registration state and mark a still-registered user as offline, corrupting
downstream presence and agent state.

Change

• Record the authoritative registration identity on every Location
  (registered_username, registered_realm), populated on register and restored
  from the DB model, so each removal path knows which identity a binding belongs
  to.
• Gate every binding-removal event on the final active binding: a new
  locations_without_active_bindings helper re-checks has_active_bindings per
  identity and only reports offline when no binding remains.
• Serialize locator mutations with their local registration events via a shared
  LocatorEventLock, so a concurrent register/unregister cannot race the
  check-then-publish pattern into emitting a stale event.
• Harden DbLocator cleanup to compare-and-delete: an expired snapshot is removed
  only when id + last_modified + expires still match, inside a transaction, with
  survivors re-verified. This keeps a concurrent REGISTER — including one from
  another process that cannot share the local mutex — from being deleted by a sweep
  after it refreshed the same row.

Tests

Regression tests cover stale/fresh, concurrent, and wildcard registration paths,
asserting that removing an expired binding while a fresher binding for the same
identity remains does not report the user offline and keeps the fresh binding
routable.