feat: 添加分类提案功能，包括提案表单和相关后端逻辑

backend/src/main/java/com/openisle/model/CategoryProposalPost.java

@@ -0,0 +1,65 @@
+package com.openisle.model;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.EnumType;
+import jakarta.persistence.Enumerated;
+import jakarta.persistence.Index;
+import jakarta.persistence.PrimaryKeyJoinColumn;
+import jakarta.persistence.Table;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+import java.time.LocalDateTime;
+
+/**
+ * A specialized post type used for proposing new categories.
+ * It reuses poll mechanics (participants, votes, endTime) by extending PollPost.
+ */
+@Entity
+@Table(name = "category_proposal_posts", indexes = {
+        @Index(name = "idx_category_proposal_posts_status", columnList = "status"),
+        @Index(name = "idx_category_proposal_posts_slug", columnList = "proposed_slug", unique = true)
+})
+@Getter
+@Setter
+@NoArgsConstructor
+@PrimaryKeyJoinColumn(name = "post_id")
+public class CategoryProposalPost extends PollPost {
+
+    @Enumerated(EnumType.STRING)
+    @Column(name = "status", nullable = false)
+    private CategoryProposalStatus proposalStatus = CategoryProposalStatus.PENDING;
+
+    @Column(name = "proposed_name", nullable = false)
+    private String proposedName;
+
+    @Column(name = "proposed_slug", nullable = false, unique = true)
+    private String proposedSlug;
+
+    @Column(name = "description")
+    private String description;
+
+    // Approval threshold as percentage (0-100), default 60
+    @Column(name = "approve_threshold", nullable = false)
+    private int approveThreshold = 60;
+
+    // Minimum number of participants required to meet quorum
+    @Column(name = "quorum", nullable = false)
+    private int quorum = 10;
+
+    // Optional voting start time (end time inherited from PollPost)
+    @Column(name = "start_at")
+    private LocalDateTime startAt;
+
+    // Snapshot of poll results at finalization (e.g., JSON)
+    @Column(name = "result_snapshot", columnDefinition = "TEXT")
+    private String resultSnapshot;
+
+    // Reason when proposal is rejected
+    @Column(name = "reject_reason")
+    private String rejectReason;
+}
+
+

backend/src/main/java/com/openisle/model/CategoryProposalStatus.java

@@ -0,0 +1,10 @@
+package com.openisle.model;
+
+public enum CategoryProposalStatus {
+    PENDING,
+    APPROVED,
+    REJECTED
+}
+
+
+

backend/src/main/java/com/openisle/model/PostType.java

@@ -4,4 +4,5 @@ public enum PostType {
   NORMAL,
   LOTTERY,
   POLL,
+  PROPOSAL
 }