feat(tickets): Foto-/Datei-Anhänge an Tickets

- Anhänge im Melde-Fenster (beim Erstellen) und direkt an bestehenden Tickets:
  Vorschaubilder, Öffnen im neuen Tab, Entfernen.
- Bytes liegen in eigener Tabelle (FeedbackAttachment, lose FeedbackId ohne FK →
  übersteht den Ingest-Wipe); GET /feedback liefert nur Metadaten (id/Name/Typ/Größe),
  die Bytes über /feedback/attachments/{id}. Größenlimit 10 MB.
- Endpoints: POST /feedback/{id}/attachments (base64), GET /feedback/attachments/{id}
  (Bytes), DELETE /feedback/attachments/{id}.

Migration FeedbackAttachments. Tests: 262 Backend grün (+Upload/Serve/Delete +Validierung),
e2e Tickets Desktop+Phone grün (+Foto-Upload), vitest 149.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

GerbilManagerWebAPI/Migrations/20260623092227_FeedbackAttachments.cs

@@ -0,0 +1,39 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace GerbilManagerWebAPI.Migrations
+{
+    /// <inheritdoc />
+    public partial class FeedbackAttachments : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "FeedbackAttachments",
+                columns: table => new
+                {
+                    Id = table.Column<Guid>(type: "uuid", nullable: false),
+                    FeedbackId = table.Column<Guid>(type: "uuid", nullable: false),
+                    FileName = table.Column<string>(type: "text", nullable: false),
+                    ContentType = table.Column<string>(type: "text", nullable: false),
+                    Size = table.Column<int>(type: "integer", nullable: false),
+                    Data = table.Column<byte[]>(type: "bytea", nullable: false),
+                    CreatedAt = table.Column<DateTimeOffset>(type: "timestamp with time zone", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_FeedbackAttachments", x => x.Id);
+                });
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "FeedbackAttachments");
+        }
+    }
+}

GerbilManagerWebAPI/Migrations/ApplicationContextModelSnapshot.cs

@@ -960,6 +960,38 @@ namespace GerbilManagerWebAPI.Migrations
                     b.ToTable("Feedback");
                 });
 
+            modelBuilder.Entity("GerbilManagerWebAPI.Models.FeedbackAttachment", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("uuid");
+
+                    b.Property<string>("ContentType")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.Property<DateTimeOffset>("CreatedAt")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<byte[]>("Data")
+                        .IsRequired()
+                        .HasColumnType("bytea");
+
+                    b.Property<Guid>("FeedbackId")
+                        .HasColumnType("uuid");
+
+                    b.Property<string>("FileName")
+                        .IsRequired()
+                        .HasColumnType("text");
+
+                    b.Property<int>("Size")
+                        .HasColumnType("integer");
+
+                    b.HasKey("Id");
+
+                    b.ToTable("FeedbackAttachments");
+                });
+
             modelBuilder.Entity("GerbilManagerWebAPI.Models.Gerbil", b =>
                 {
                     b.Property<Guid>("Id")