Add demo logic for landing

src/main/java/ru/soune/nocopy/entity/demo/DemoSession.java

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+package ru.soune.nocopy.entity.demo;
+
+import jakarta.persistence.*;
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+import java.time.LocalDateTime;
+
+@Entity
+@Table(name = "demo_sessions")
+@Data
+@NoArgsConstructor
+@AllArgsConstructor
+@Builder
+public class DemoSession {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.UUID)
+    private String sessionId;
+
+    @Column(nullable = false)
+    private String ipAddress;
+
+    @Column(nullable = false)
+    private String fileName;
+
+    @Column(nullable = false)
+    private String fileId;
+
+    @Column(nullable = false)
+    private String s3Path;
+
+    @Column(nullable = false)
+    private Long fileSize;
+
+    @Column(nullable = false)
+    private String fileType; // "image", "document", "audio"
+
+    @Column(nullable = false)
+    @Enumerated(EnumType.STRING)
+    private DemoSessionStatus status; // CREATED, PROCESSING, COMPLETED, FAILED
+
+    @Column
+    private String mimeType;
+
+    @Column
+    private Integer searchResultsCount;
+
+    @Column(nullable = false, updatable = false)
+    private LocalDateTime createdAt;
+
+    @Column(nullable = false)
+    private LocalDateTime updatedAt;
+
+    @Column(nullable = false)
+    private LocalDateTime expiresAt; // 24 часа с момента создания
+
+    @PrePersist
+    protected void onCreate() {
+        createdAt = LocalDateTime.now();
+        updatedAt = LocalDateTime.now();
+        expiresAt = LocalDateTime.now().plusHours(24);
+        status = DemoSessionStatus.CREATED;
+    }
+
+    @PreUpdate
+    protected void onUpdate() {
+        updatedAt = LocalDateTime.now();
+    }
+}

src/main/java/ru/soune/nocopy/entity/demo/DemoSessionStatus.java

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+package ru.soune.nocopy.entity.demo;
+
+public enum DemoSessionStatus {
+    CREATED,
+    PROCESSING,
+    COMPLETED,
+    FAILED,
+    EXPIRED,
+    CLEANED
+}