feat: demo data seeder — one-click setup for new cafés

Adds POST /api/cafes/{cafeId}/demo/seed (owner-only) that seeds:
- 9% default VAT tax
- 7 menu categories + 59+ items via DemoMenuSeeder
- 15 inventory ingredients (coffee shop staples)
- 10 tables across 3 floors on the first active branch

Frontend DemoDataBanner appears on menu, tables, and inventory
pages when the café is completely empty, so owners can populate
demo data in one click instead of entering everything manually.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Meezi.API/Controllers/DemoSeedController.cs

@@ -0,0 +1,33 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using Meezi.API.Services;
+using Meezi.Core.Interfaces;
+using Meezi.Shared;
+
+namespace Meezi.API.Controllers;
+
+[Route("api/cafes/{cafeId}/demo")]
+[Authorize]
+public class DemoSeedController : CafeApiControllerBase
+{
+    private readonly IDemoSeedService _demoSeed;
+
+    public DemoSeedController(IDemoSeedService demoSeed)
+    {
+        _demoSeed = demoSeed;
+    }
+
+    /// <summary>Seeds demo menu, tables, and inventory for any café. Owner-only.</summary>
+    [HttpPost("seed")]
+    public async Task<IActionResult> Seed(
+        string cafeId,
+        ITenantContext tenant,
+        CancellationToken ct)
+    {
+        if (EnsureCafeAccess(cafeId, tenant) is { } denied) return denied;
+        if (EnsureOwner(tenant) is { } ownerDenied) return ownerDenied;
+
+        var result = await _demoSeed.SeedAsync(cafeId, ct);
+        return Ok(new ApiResponse<DemoSeedResult>(true, result));
+    }
+}