feat: 登录验证 + 邀请码注册 + 用量监控后台

- 新增用户认证系统（登录/登出/邀请码注册）
- 新增管理后台（邀请码管理/用户管理/使用记录/用量统计）
- 合成接口加登录验证，每次调用记录用户和稿件内容
- SQLite存储用户数据和使用日志
- 默认admin账号: simonkoson

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/db.py

@@ -0,0 +1,69 @@
+import sqlite3
+import os
+from werkzeug.security import generate_password_hash
+
+DB_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), '..', 'data')
+DB_PATH = os.path.join(DB_DIR, 'voice.db')
+
+
+def get_db():
+    """获取数据库连接"""
+    conn = sqlite3.connect(DB_PATH)
+    conn.row_factory = sqlite3.Row
+    conn.execute("PRAGMA journal_mode=WAL")
+    return conn
+
+
+def init_db():
+    """初始化数据库表 + 创建默认 admin 账号"""
+    os.makedirs(DB_DIR, exist_ok=True)
+    conn = get_db()
+    cursor = conn.cursor()
+
+    cursor.executescript('''
+        CREATE TABLE IF NOT EXISTS users (
+            id            INTEGER PRIMARY KEY AUTOINCREMENT,
+            username      TEXT UNIQUE NOT NULL,
+            display_name  TEXT NOT NULL,
+            password_hash TEXT NOT NULL,
+            role          TEXT DEFAULT 'editor',
+            is_active     INTEGER DEFAULT 1,
+            created_at    DATETIME DEFAULT CURRENT_TIMESTAMP
+        );
+
+        CREATE TABLE IF NOT EXISTS invite_codes (
+            id            INTEGER PRIMARY KEY AUTOINCREMENT,
+            code          TEXT UNIQUE NOT NULL,
+            editor_name   TEXT NOT NULL,
+            is_used       INTEGER DEFAULT 0,
+            used_by_user_id INTEGER,
+            created_at    DATETIME DEFAULT CURRENT_TIMESTAMP,
+            used_at       DATETIME
+        );
+
+        CREATE TABLE IF NOT EXISTS usage_logs (
+            id            INTEGER PRIMARY KEY AUTOINCREMENT,
+            user_id       INTEGER NOT NULL,
+            username      TEXT NOT NULL,
+            display_name  TEXT NOT NULL,
+            text_content  TEXT NOT NULL,
+            text_length   INTEGER NOT NULL,
+            engine        TEXT NOT NULL,
+            emotion       TEXT,
+            created_at    DATETIME DEFAULT CURRENT_TIMESTAMP,
+            FOREIGN KEY (user_id) REFERENCES users(id)
+        );
+    ''')
+
+    # 创建默认 admin 账号（如不存在）
+    existing = cursor.execute(
+        'SELECT id FROM users WHERE username = ?', ('simonkoson',)
+    ).fetchone()
+    if not existing:
+        cursor.execute(
+            'INSERT INTO users (username, display_name, password_hash, role) VALUES (?, ?, ?, ?)',
+            ('simonkoson', '蓝皓', generate_password_hash('liutong65'), 'admin')
+        )
+
+    conn.commit()
+    conn.close()