feat: P0 commercial readiness — password security, legal, account management

Backend changes for Phase A of v1.0 commercial launch:

P0-3: Password Security
- Remove hardcoded admin/123456 defaults from 34+ bootstrap scripts
- Add PLATFORM_USERNAME/PLATFORM_PASSWORD env vars to config
- Add PUT /api/v1/auth/change-password endpoint (requires old password)

P0-1: User Agreement & Privacy Policy
- Add agreed_terms/agreed_terms_version/agreed_terms_at to User model
- New GET /api/v1/legal/privacy and GET /api/v1/legal/terms endpoints
- New POST /api/v1/legal/agree endpoint to record consent
- Register endpoint now validates agreed_terms must be True

P0-6: Account Management
- Add phone/status/is_email_verified to User model
- Add DELETE /api/v1/auth/account for account deletion (with password confirmation)
- Add PUT /api/v1/auth/phone for phone binding
- Reject authentication for deleted accounts (status=deleted)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/scripts/set_admin.py

@@ -19,8 +19,12 @@ def set_admin():
         print("=" * 60)
         print()
         
-        # 查找admin用户
-        admin_user = db.query(User).filter(User.username == "admin").first()
+        target_username = os.getenv("PLATFORM_USERNAME")
+        if not target_username:
+            print("❌ 请设置 PLATFORM_USERNAME 环境变量指定要提升为管理员的用户名")
+            return
+        # 查找目标用户
+        admin_user = db.query(User).filter(User.username == target_username).first()
         
         if not admin_user:
             print("❌ 未找到admin用户，请先创建admin用户")