refactor: BindSentryContext to AuthScopeContextListener for auth-scope tags

Sentry-context binding split into two responsibilities:

- Route-scope (app, http.method, route_name) stays in middleware on
  the api group as BindSentryRouteContext — works on every request,
  no auth required.
- Auth-scope (user_id, actor_type) moves to AuthScopeContextListener
  on Illuminate\Auth\Events\Authenticated — works on every
  authentication mechanism (Sanctum, portal-tokens, future
  authenticators) without per-route middleware-attachment. Listener
  also augments Log::withContext with user_id (closes OBS-2).

Architecturally fault-preventing rather than fault-detecting: new
authenticated route groups need no separate sentry.context aliasing,
so silent observability gaps are no longer possible (closes OBS-3).

Impersonation tagging is co-located with HandleImpersonation: after
the user-swap, the middleware re-tags Sentry scope with the target
user_id/actor_type and adds impersonation.active /
impersonation.impersonator_user_id / impersonation.session_id. The
Authenticated event fires for the admin (Sanctum's natural flow),
the listener tags the admin, then HandleImpersonation overwrites
post-swap.

Files renamed:
- BindSentryContext -> BindSentryRouteContext (route-scope only)
- BindSentryContextTest -> BindSentryRouteContextTest (4 cases)

Files added:
- AuthScopeContextListener
- AuthScopeContextListenerTest (6 cases)

bootstrap/app.php drops the sentry.context alias and prepends
BindSentryRouteContext to the api group. routes/api.php drops every
sentry.context middleware string from auth:sanctum groups.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/routes/api.php

@@ -120,7 +120,7 @@ Route::middleware('throttle:30,1')->group(function (): void {
 
 // Platform Admin routes
 Route::prefix('admin')
-    ->middleware(['auth:sanctum', 'impersonation', 'sentry.context', 'role:super_admin'])
+    ->middleware(['auth:sanctum', 'impersonation', 'role:super_admin'])
     ->name('admin.')
     ->group(function () {
         // Organisations
@@ -155,7 +155,7 @@ Route::prefix('admin')
     });
 
 // Protected routes
-Route::middleware(['auth:sanctum', 'impersonation', 'sentry.context'])->group(function () {
+Route::middleware(['auth:sanctum', 'impersonation'])->group(function () {
     // Impersonation (stop — accessible by impersonated user, not just super_admin)
     Route::post('admin/stop-impersonation', [AdminImpersonationController::class, 'stop']);