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/debug_cache_query.py

@@ -1,3 +1,4 @@
+import os
 """单次执行后打印 cache-query 与 llm-unified 输出，用于排查记忆。"""
 import json
 import requests
@@ -9,7 +10,7 @@ MSG = "我叫李小红"
 
 r = requests.post(
     B + "/api/v1/auth/login",
-    data={"username": "admin", "password": "123456"},
+    data={"username": os.getenv("PLATFORM_USERNAME"), "password": os.getenv("PLATFORM_PASSWORD")},
     headers={"Content-Type": "application/x-www-form-urlencoded"},
     timeout=15,
 )