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/generate_batch_agents.py

@@ -524,7 +524,7 @@ def generate_creative_writing_agent(db: Session, user: User):
     }
 
 
-def generate_batch_agents(db: Session, username: str = "admin"):
+def generate_batch_agents(db: Session, username: str | None = None):
     """批量生成Agent"""
     print("=" * 60)
     print("批量生成Agent")
@@ -617,7 +617,7 @@ def main():
     """主函数"""
     db = SessionLocal()
     try:
-        generate_batch_agents(db, username="admin")
+        generate_batch_agents(db, username=os.getenv("PLATFORM_USERNAME"))
     finally:
         db.close()