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/AppDbContext.cs

@@ -85,6 +85,9 @@ public class AppDbContext : DbContext
     // Idempotency keys for safe retry of offline-replayed writes.
     public DbSet<IdempotencyRecord> IdempotencyRecords => Set<IdempotencyRecord>();
 
+    // Uploaded files, recorded for content-hash de-duplication and a media library.
+    public DbSet<MediaAsset> MediaAssets => Set<MediaAsset>();
+
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
         base.OnModelCreating(modelBuilder);
@@ -113,6 +116,20 @@ public class AppDbContext : DbContext
             e.HasQueryFilter(x => x.DeletedAt == null);
         });
 
+        modelBuilder.Entity<MediaAsset>(e =>
+        {
+            e.HasKey(x => x.Id);
+            // Dedup lookups: same content within a scope (café or platform).
+            e.HasIndex(x => new { x.CafeId, x.ContentHash });
+            e.Property(x => x.CafeId).HasMaxLength(64);
+            e.Property(x => x.ContentHash).HasMaxLength(64).IsRequired();
+            e.Property(x => x.ContentType).HasMaxLength(100).IsRequired();
+            e.Property(x => x.Url).HasMaxLength(500).IsRequired();
+            e.Property(x => x.Kind).HasMaxLength(40).IsRequired();
+            e.Property(x => x.OriginalFileName).HasMaxLength(260);
+            e.HasQueryFilter(x => x.DeletedAt == null);
+        });
+
         modelBuilder.Entity<Cafe>(e =>
         {
             e.HasKey(x => x.Id);

src/Meezi.Infrastructure/Data/Migrations/20260602184236_AddMediaAssets.cs

@@ -0,0 +1,47 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Meezi.Infrastructure.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddMediaAssets : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "MediaAssets",
+                columns: table => new
+                {
+                    Id = table.Column<string>(type: "text", nullable: false),
+                    CafeId = table.Column<string>(type: "character varying(64)", maxLength: 64, nullable: true),
+                    ContentHash = table.Column<string>(type: "character varying(64)", maxLength: 64, nullable: false),
+                    SizeBytes = table.Column<long>(type: "bigint", nullable: false),
+                    ContentType = table.Column<string>(type: "character varying(100)", maxLength: 100, nullable: false),
+                    Url = table.Column<string>(type: "character varying(500)", maxLength: 500, nullable: false),
+                    Kind = table.Column<string>(type: "character varying(40)", maxLength: 40, nullable: false),
+                    OriginalFileName = table.Column<string>(type: "character varying(260)", maxLength: 260, nullable: true),
+                    CreatedAt = table.Column<DateTime>(type: "timestamp with time zone", nullable: false),
+                    DeletedAt = table.Column<DateTime>(type: "timestamp with time zone", nullable: true)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_MediaAssets", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_MediaAssets_CafeId_ContentHash",
+                table: "MediaAssets",
+                columns: new[] { "CafeId", "ContentHash" });
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "MediaAssets");
+        }
+    }
+}

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")