fix: stop returning model api keys and voice credentials to the frontend

[jeffwu-1999]

变更说明 / What Changed

修改模型时 api-key 等凭证返回前端。模型列表接口此前已剥离api_key，但仍有两个泄露路径，本 PR 一并堵住：

• config: /config/load_config (build_model_config) no longer echoes the
  stored api_key, nor the STT/TTS model_appid / access_token pair. The
  config page never displays or resubmits these fields from this payload,
  so nothing breaks by omitting them
• model: /model/* list responses (_sanitize_model_credentials) now also
  strip model_appid / access_token — the full auth material for Volcano
  Engine STT/TTS models, previously returned in cleartext
• update: empty-string model_appid / access_token are dropped from the
  update payload instead of overwriting stored values, mirroring the
  existing "empty api_key means keep" contract the edit dialog relies on
• test: cover the extended sanitization (nested dict, HTTP responses) and
  the keep-existing update semantics; update assertions that previously
  expected raw keys

验证 / Verification

• pytest test/backend/app/test_model_managment_app.py + test_model_rbac.py — 96 passed
• pytest test/backend/services/test_config_sync_service.py — 58 passed
• pytest test/backend/services/test_config_sync_service_voice.py — 4 passed
• pytest test/backend/services/test_model_management_service.py — 106 passed
• pytest test/backend/services/test_model_health_service.py — 58 passed

影响面 / Impact

• 所有角色的 /config/load_config 与 /model/* 列表响应不再包含任何凭证
• 编辑模型弹窗「留空 = 保留原值」语义对 apiKey / modelAppid / accessToken 三者一致成立
• 语音模型连通性检查不受影响（health 探测在后端内部读取模型记录，不经 HTTP 响应）

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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