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.Infrastructure/Data/Migrations/AppDbContextModelSnapshot.cs

@@ -1308,6 +1308,55 @@ namespace Meezi.Infrastructure.Data.Migrations
                     b.ToTable("LeaveRequests");
                 });
 
+            modelBuilder.Entity("Meezi.Core.Entities.MediaAsset", b =>
+                {
+                    b.Property<string>("Id")
+                        .HasColumnType("text");
+
+                    b.Property<string>("CafeId")
+                        .HasMaxLength(64)
+                        .HasColumnType("character varying(64)");
+
+                    b.Property<string>("ContentHash")
+                        .IsRequired()
+                        .HasMaxLength(64)
+                        .HasColumnType("character varying(64)");
+
+                    b.Property<string>("ContentType")
+                        .IsRequired()
+                        .HasMaxLength(100)
+                        .HasColumnType("character varying(100)");
+
+                    b.Property<DateTime>("CreatedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<DateTime?>("DeletedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<string>("Kind")
+                        .IsRequired()
+                        .HasMaxLength(40)
+                        .HasColumnType("character varying(40)");
+
+                    b.Property<string>("OriginalFileName")
+                        .HasMaxLength(260)
+                        .HasColumnType("character varying(260)");
+
+                    b.Property<long>("SizeBytes")
+                        .HasColumnType("bigint");
+
+                    b.Property<string>("Url")
+                        .IsRequired()
+                        .HasMaxLength(500)
+                        .HasColumnType("character varying(500)");
+
+                    b.HasKey("Id");
+
+                    b.HasIndex("CafeId", "ContentHash");
+
+                    b.ToTable("MediaAssets");
+                });
+
             modelBuilder.Entity("Meezi.Core.Entities.MenuCategory", b =>
                 {
                     b.Property<string>("Id")