refactor: 法治日报改为法治网 + 删除无用conversations/messages表

- 删除 conversations 和 messages 表（未被代码使用）
- 迁移文件：000017_drop_conversations_messages.up/down.sql
- 种子文件改名：seed_fazhiribao_* → seed_fazhiwang_*
- 组织数据：法治日报 → 法治网（slug: fazhiwang）
- 用户数据：法治日报管理员等 → 法治网管理员等
- 演示数据重构：使用 app_usage_logs 而非 conversations/messages
- 本地数据已更新：1个组织、15个应用、3个用户、5条示例对话

server/migrations/seed_fazhiwang_users.sql

@@ -0,0 +1,39 @@
+-- 法治网机构用户种子数据
+-- 密码统一为 admin123
+-- 可重复执行（ON CONFLICT DO NOTHING）
+
+DO $$
+DECLARE
+  pwd_hash TEXT;
+  fazhi_org UUID := 'a0000000-0000-0000-0000-00000000000a';
+BEGIN
+  -- 获取已有用户的密码hash（admin123）
+  SELECT password_hash INTO pwd_hash FROM users WHERE email = 'admin@govai.gov.cn';
+
+  -- 法治网机构管理员（排序第一位，作为登录页默认账号）
+  INSERT INTO organizations (id, name, slug, short_name, description, sort_order, is_active) VALUES
+    (fazhi_org, '法治网', 'fazhiwang', '法治网',
+     '法治网官方平台，提供法治资讯、政策解读、法律服务与智能应用',
+     1, true)
+  ON CONFLICT (id) DO NOTHING;
+
+  -- 显式重排所有机构，确保法治网稳定排在第一位、其余机构顺延（幂等）
+  UPDATE organizations SET sort_order = 1  WHERE slug = 'fazhiwang';
+  UPDATE organizations SET sort_order = 2  WHERE slug = 'keji';
+  UPDATE organizations SET sort_order = 3  WHERE slug = 'gongan';
+  UPDATE organizations SET sort_order = 4  WHERE slug = 'fagai';
+  UPDATE organizations SET sort_order = 5  WHERE slug = 'jiaoyu';
+  UPDATE organizations SET sort_order = 6  WHERE slug = 'renshe';
+  UPDATE organizations SET sort_order = 7  WHERE slug = 'caizheng';
+  UPDATE organizations SET sort_order = 8  WHERE slug = 'zhujian';
+  UPDATE organizations SET sort_order = 9  WHERE slug = 'shijianju';
+  UPDATE organizations SET sort_order = 10 WHERE slug = 'lvsuo';
+
+  -- 法治网机构用户
+  INSERT INTO users (id, name, email, password_hash, role, status, org_id) VALUES
+    (gen_random_uuid(), '法治网管理员', 'fazhiwang@govai.gov.cn', pwd_hash, 'admin', 'active', fazhi_org),
+    (gen_random_uuid(), '法治网编辑', 'fazhibian@govai.gov.cn', pwd_hash, 'creator', 'active', fazhi_org),
+    (gen_random_uuid(), '法治网记者', 'fazhijizhe@govai.gov.cn', pwd_hash, 'user', 'active', fazhi_org)
+  ON CONFLICT (email) DO NOTHING;
+
+END $$;