feat(media): content-hash dedup for uploads + media-library endpoint

Uploads previously wrote every file to disk with a fresh GUID name, so the
same image uploaded twice produced two identical files. Now:

- New MediaAsset table records each stored upload (SHA-256 hash, size, type,
  url, kind, scope) + migration. Indexed on (CafeId, ContentHash).
- MediaStorageService computes the content hash on upload; if an identical file
  already exists for that café it returns the existing URL instead of writing a
  duplicate (covers images, videos, 3D models). Dedup lookup/record run via a
  scoped DbContext (the service is a singleton) and never block an upload on
  failure.
- GET /api/cafes/{cafeId}/media lists the café's library (newest first, optional
  ?kind=) so the UI can let users pick an existing file instead of re-uploading.

86 API tests pass.

src/Meezi.Core/Entities/MediaAsset.cs

@@ -0,0 +1,27 @@
+namespace Meezi.Core.Entities;
+
+/// <summary>
+/// A stored upload, recorded so identical files can be de-duplicated and reused
+/// instead of written to disk again. Files with the same content hash within a
+/// scope (café, or platform when <see cref="CafeId"/> is null) share one stored file.
+/// </summary>
+public class MediaAsset : BaseEntity
+{
+    /// <summary>Owning café, or null for platform-level (admin) uploads.</summary>
+    public string? CafeId { get; set; }
+
+    /// <summary>SHA-256 of the file content, lowercase hex.</summary>
+    public string ContentHash { get; set; } = string.Empty;
+
+    public long SizeBytes { get; set; }
+
+    public string ContentType { get; set; } = string.Empty;
+
+    /// <summary>Public URL/path the file is served from (e.g. /uploads/{cafeId}/{name}).</summary>
+    public string Url { get; set; } = string.Empty;
+
+    /// <summary>Logical kind/prefix: menu_img, logo, cover, gallery, review, menu_3d, blog, ...</summary>
+    public string Kind { get; set; } = string.Empty;
+
+    public string? OriginalFileName { get; set; }
+}