fix: auto-create default branch on cafe registration + backfill existing

- VerifyRegisterAsync: create a Branch named after the café alongside
  the Café and Owner, so new owners can use the dashboard immediately
  without hitting the "select a branch" gate
- PlatformDataSeeder: EnsureDefaultBranchesAsync runs on every boot and
  creates a default branch for any existing café that has none (covers
  cafés registered before this fix)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/Meezi.API/Services/AuthService.cs

@@ -367,6 +367,15 @@ public class AuthService : IAuthService
             PlanTier = PlanTier.Free,
         };
 
+        // Auto-create a default main branch so the owner can start using the
+        // dashboard immediately without hitting the "select a branch" gate.
+        var defaultBranch = new Branch
+        {
+            CafeId   = cafe.Id,
+            Name     = cafeName,
+            IsActive = true,
+        };
+
         var owner = new Employee
         {
             CafeId = cafe.Id,
@@ -376,6 +385,7 @@ public class AuthService : IAuthService
         };
 
         _db.Cafes.Add(cafe);
+        _db.Branches.Add(defaultBranch);
         _db.Employees.Add(owner);
         await _db.SaveChangesAsync(cancellationToken);