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/e2e_platform_capability_smoke_testclient.py

@@ -23,7 +23,7 @@ def main() -> int:
     c = TestClient(app)
     r = c.post(
         "/api/v1/auth/login",
-        data={"username": os.getenv("PLATFORM_USERNAME", "admin"), "password": os.getenv("PLATFORM_PASSWORD", "123456")},
+        data={"username": os.getenv("PLATFORM_USERNAME"), "password": os.getenv("PLATFORM_PASSWORD")},
         headers={"Content-Type": "application/x-www-form-urlencoded"},
     )
     if r.status_code != 200: