refactor: 优化数据库迁移文件约束和注释

**迁移文件优化**：
- 001_users.up.sql: 添加完整表和列注释，说明密码哈希算法 (bcrypt) 和 token 哈希算法 (SHA-256)
- 002_messages.up.sql: 收紧 role 字段至 VARCHAR(10)，添加 tokens_used 非负约束，补充完整注释
- 003_sessions.up.sql: 收紧 title 字段至 VARCHAR(100) 并添加长度约束 (1-100)，详细说明 config JSONB 结构
- 004_user_scenarios.up.sql: 修复 greeting 字段冲突 (VARCHAR(500))，扩大 icon 至 VARCHAR(20) 支持复合 Emoji，prompt 改为 TEXT 无上限，优化约束逻辑

**后端代码修改**：
- auth.go: 移除用户名最小长度限制 (3 字符)，仅保留最大长度 64

**前端国际化修改**：
- 更新三种语言的登录/注册表单 placeholder 文本，移除字符长度要求提示
  - zh-CN: "请输入用户名" / "请输入密码"
  - en-US: "Enter username" / "Enter password"
  - ja-JP: "ユーザー名を入力" / "パスワードを入力"

**删除文件**：
- 移除临时修复迁移文件 (005_fix_user_scenarios_description.*)
- 删除临时诊断脚本 (fix_user_scenarios.sql, check_constraints.sql)

所有约束修改都是放宽限制，不影响现有数据。

frontend/src/lib/i18n/en-US.ts

@@ -155,9 +155,9 @@ export const enUS: TranslationMap = {
   "auth.login": "Sign In",
   "auth.register": "Sign Up",
   "auth.username": "Username",
-  "auth.username.placeholder": "3-64 characters",
+  "auth.username.placeholder": "Enter username",
   "auth.password": "Password",
-  "auth.password.placeholder": "8-72 characters",
+  "auth.password.placeholder": "Enter password",
   "auth.submitting": "Please wait...",
   "auth.noAccount": "Don't have an account?",
   "auth.hasAccount": "Already have an account?",

frontend/src/lib/i18n/ja-JP.ts

@@ -155,9 +155,9 @@ export const jaJP: TranslationMap = {
   "auth.login": "ログイン",
   "auth.register": "新規登録",
   "auth.username": "ユーザー名",
-  "auth.username.placeholder": "3〜64文字",
+  "auth.username.placeholder": "ユーザー名を入力",
   "auth.password": "パスワード",
-  "auth.password.placeholder": "8〜72文字",
+  "auth.password.placeholder": "パスワードを入力",
   "auth.submitting": "お待ちください...",
   "auth.noAccount": "アカウントをお持ちでないですか？",
   "auth.hasAccount": "すでにアカウントをお持ちですか？",

frontend/src/lib/i18n/zh-CN.ts

@@ -155,9 +155,9 @@ export const zhCN: TranslationMap = {
   "auth.login": "登录",
   "auth.register": "注册",
   "auth.username": "用户名",
-  "auth.username.placeholder": "3-64 个字符",
+  "auth.username.placeholder": "请输入用户名",
   "auth.password": "密码",
-  "auth.password.placeholder": "8-72 个字符",
+  "auth.password.placeholder": "请输入密码",
   "auth.submitting": "请稍候...",
   "auth.noAccount": "还没有账号？",
   "auth.hasAccount": "已有账号？",