feat(share): reuse existing share and direct links per file

backend/database.js

@@ -1354,6 +1354,31 @@ const ShareDB = {
     return db.prepare('SELECT * FROM shares WHERE id = ?').get(id);
   },
 
+  // 按目标路径查找已存在分享（用于复用）
+  findExistingByTarget(userId, options = {}) {
+    const {
+      share_type = 'file',
+      file_path = '',
+      storage_type = 'oss'
+    } = options;
+
+    return db.prepare(`
+      SELECT *
+      FROM shares
+      WHERE user_id = ?
+        AND share_type = ?
+        AND share_path = ?
+        AND COALESCE(storage_type, 'oss') = ?
+      ORDER BY id DESC
+      LIMIT 1
+    `).get(
+      userId,
+      share_type,
+      file_path,
+      storage_type || 'oss'
+    );
+  },
+
   // 验证分享密码
   verifyPassword(plainPassword, hashedPassword) {
     return bcrypt.compareSync(plainPassword, hashedPassword);
@@ -1492,6 +1517,28 @@ const DirectLinkDB = {
     return db.prepare('SELECT * FROM direct_links WHERE id = ?').get(id);
   },
 
+  // 按目标路径查找已存在直链（用于复用）
+  findExistingByTarget(userId, options = {}) {
+    const {
+      file_path = '',
+      storage_type = 'oss'
+    } = options;
+
+    return db.prepare(`
+      SELECT *
+      FROM direct_links
+      WHERE user_id = ?
+        AND file_path = ?
+        AND COALESCE(storage_type, 'oss') = ?
+      ORDER BY id DESC
+      LIMIT 1
+    `).get(
+      userId,
+      file_path,
+      storage_type || 'oss'
+    );
+  },
+
   getUserLinks(userId) {
     return db.prepare(`
       SELECT *