fix(backend): 修复 DeepSeek 模型列表为空 & whisper 下载不走代理 (#417)

问题1 — DeepSeek 模型下拉为空：
provider 的 /models 调用失败时，get_model_list 吞掉异常返回 []，
get_all_models_by_id 再对 [] 取 .data 触发 AttributeError 又被吞，
最终接口返回 {"code":0,"msg":"success","data":[]}，把失败伪装成空成功，
用户看不到任何原因。
- 捕获并回传真实错误，不再二次吞
- 新增 model_fallback.normalize_models 兼容 SyncPage/list/dict，绝不再 .data 崩
- 内置供应商提供已知模型兜底清单（DeepSeek→deepseek-chat/deepseek-reasoner，
  Qwen→qwen-plus 等），动态拿不到时回退，保证下拉非空；动态可用时仍以动态为准

问题2 — whisper 模型下载不走代理（Docker 里代理没生效）：
snapshot_download 既不读 UI 配的代理，HF_ENDPOINT 又固定 hf-mirror.com。
- ProxyConfigManager.apply_to_env() 把生效代理 export 到 HTTP(S)_PROXY/ALL_PROXY，
  huggingface_hub 即可复用；在每次下载前与启动时应用（覆盖转写按需下载）
- 网络类下载报错翻译成可操作提示（配代理 / 改 HF_ENDPOINT / 检查容器外网）
- .env.example 补充 HF_ENDPOINT 与代理覆盖说明

新增 tests/test_model_fallback.py、tests/test_proxy_apply_env.py（14 用例）

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/tests/test_model_fallback.py

@@ -0,0 +1,89 @@
+import importlib.util
+import pathlib
+import unittest
+
+ROOT = pathlib.Path(__file__).resolve().parents[1]
+MODULE_PATH = ROOT / "app" / "services" / "model_fallback.py"
+spec = importlib.util.spec_from_file_location("model_fallback", MODULE_PATH)
+if spec is None or spec.loader is None:
+    raise ImportError("model_fallback module spec not found")
+model_fallback = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(model_fallback)
+
+builtin_fallback_models = model_fallback.builtin_fallback_models
+normalize_models = model_fallback.normalize_models
+as_model_dicts = model_fallback.as_model_dicts
+
+
+class FakeModel:
+    """模拟 openai SDK 的 Model（pydantic）对象。"""
+
+    def __init__(self, mid, created=None, owned_by="x"):
+        self.id = mid
+        self.created = created
+        self.object = "model"
+        self.owned_by = owned_by
+
+    def dict(self):
+        return {"id": self.id, "created": self.created, "object": self.object, "owned_by": self.owned_by}
+
+
+class FakeSyncPage:
+    def __init__(self, data):
+        self.data = data
+
+
+class TestBuiltinFallbackModels(unittest.TestCase):
+    def test_deepseek_by_id_has_fallback(self):
+        # issue #417：DeepSeek /models 不稳定，必须有兜底清单
+        models = builtin_fallback_models({"id": "deepseek", "name": "DeepSeek"})
+        self.assertIn("deepseek-chat", models)
+        self.assertIn("deepseek-reasoner", models)
+
+    def test_match_by_name_case_insensitive(self):
+        models = builtin_fallback_models({"id": "whatever-uuid", "name": "deepseek"})
+        self.assertIn("deepseek-chat", models)
+
+    def test_unknown_provider_returns_empty(self):
+        self.assertEqual(builtin_fallback_models({"id": "nope", "name": "nope"}), [])
+
+    def test_none_provider_returns_empty(self):
+        self.assertEqual(builtin_fallback_models(None), [])
+
+
+class TestNormalizeModels(unittest.TestCase):
+    def test_syncpage_with_models(self):
+        page = FakeSyncPage([FakeModel("deepseek-chat"), FakeModel("deepseek-reasoner")])
+        out = normalize_models(page)
+        self.assertEqual([m["id"] for m in out], ["deepseek-chat", "deepseek-reasoner"])
+
+    def test_plain_list_of_models(self):
+        out = normalize_models([FakeModel("a"), FakeModel("b")])
+        self.assertEqual([m["id"] for m in out], ["a", "b"])
+
+    def test_empty_list_does_not_raise(self):
+        # 关键回归：旧代码对 [] 取 .data 会 AttributeError
+        self.assertEqual(normalize_models([]), [])
+
+    def test_list_of_dicts(self):
+        out = normalize_models([{"id": "x", "object": "model"}])
+        self.assertEqual(out[0]["id"], "x")
+
+    def test_drops_entries_without_id(self):
+        out = normalize_models([{"object": "model"}, {"id": "ok"}])
+        self.assertEqual([m["id"] for m in out], ["ok"])
+
+    def test_none_returns_empty(self):
+        self.assertEqual(normalize_models(None), [])
+
+
+class TestAsModelDicts(unittest.TestCase):
+    def test_shape_matches_sdk(self):
+        out = as_model_dicts(["deepseek-chat"], owned_by="DeepSeek")
+        self.assertEqual(out[0]["id"], "deepseek-chat")
+        self.assertEqual(out[0]["object"], "model")
+        self.assertEqual(out[0]["owned_by"], "DeepSeek")
+
+
+if __name__ == "__main__":
+    unittest.main()