fix: P0-2 OSS 凭证启动验证 + 诊断日志

- config.py: 非开发环境 OSS_ACCESS_KEY_ID/SECRET 为空时启动失败（fail-fast）
- storage.py: 新增 diagnose() 方法，启动时输出 OSS 配置状态
- 新增 7 个单元测试覆盖凭证验证和诊断逻辑

注意：staging 实际已有 OSS 凭证配置，预签名 URL 可正常生成。
真正问题是 sign_url 返回 HTTP 而非 HTTPS，后续修复。

apps/api/app/config.py

@@ -79,6 +79,27 @@ class Settings(BaseSettings):
     OSS_ACCESS_KEY_ID: str = ""
     OSS_ACCESS_KEY_SECRET: str = ""
     OSS_BUCKET_NAME: str = "xiaoxia-autocut"
+
+    @field_validator("OSS_ACCESS_KEY_ID", mode="before")
+    @classmethod
+    def validate_oss_access_key_id(cls, v):
+        if (v is None or v == "") and os.getenv("APP_ENV", "development") != "development":
+            raise ValueError(
+                "OSS_ACCESS_KEY_ID must be set via environment variable in non-development environments. "
+                "Check the server .env file (e.g. /var/lib/xiaoxia-saas-staging/.env)."
+            )
+        return v or ""
+
+    @field_validator("OSS_ACCESS_KEY_SECRET", mode="before")
+    @classmethod
+    def validate_oss_access_key_secret(cls, v):
+        if (v is None or v == "") and os.getenv("APP_ENV", "development") != "development":
+            raise ValueError(
+                "OSS_ACCESS_KEY_SECRET must be set via environment variable in non-development environments. "
+                "Check the server .env file (e.g. /var/lib/xiaoxia-saas-staging/.env)."
+            )
+        return v or ""
+
     OSS_DIRECT_UPLOAD_MAX_MB: int = Field(
         default=2000,
         validation_alias=AliasChoices("OSS_DIRECT_UPLOAD_MAX_MB", "MAX_UPLOAD_SIZE_MB"),

apps/api/app/core/storage.py

@@ -64,6 +64,26 @@ class OSSStorageService:
         self.access_key_secret = settings.OSS_ACCESS_KEY_SECRET
         self.endpoint = settings.OSS_ENDPOINT
 
+    def diagnose(self) -> None:
+        """启动诊断：输出 OSS 配置状态，帮助排查预签名 URL 问题。"""
+        key_id_display = (
+            f"{self.access_key_id[:4]}...{self.access_key_id[-4:]}" if len(self.access_key_id) > 8 else "(empty)"
+        )
+        logger.info(
+            "[OSS诊断] endpoint=%s bucket_name=%s access_key_id=%s",
+            self.endpoint,
+            self.bucket_name,
+            key_id_display,
+        )
+        if self.bucket is None:
+            logger.error(
+                "[OSS诊断] ❌ bucket=None — 预签名URL不可用！"
+                "原因: OSS_ACCESS_KEY_ID/OSS_ACCESS_KEY_SECRET 未配置或 oss2 未安装。"
+                "请检查服务器 .env 文件（如 /var/lib/xiaoxia-saas-staging/.env）"
+            )
+        else:
+            logger.info("[OSS诊断] ✅ bucket 已配置，预签名URL可用")
+
     def _is_local_generated_url(self, storage_key_or_url: str) -> bool:
         parsed = urlparse(storage_key_or_url)
         path = parsed.path if parsed.scheme else storage_key_or_url
@@ -167,8 +187,7 @@ class OSSStorageService:
             if self._is_local_generated_url(storage_key_or_url):
                 return storage_key_or_url
             logger.warning(
-                "get_download_url: OSS bucket not configured, returning raw URL. "
-                "storage_key_or_url=%s",
+                "get_download_url: OSS bucket not configured, returning raw URL. " "storage_key_or_url=%s",
                 storage_key_or_url[:200],
             )
             return self.get_url(self._normalize_storage_key(storage_key_or_url))
@@ -184,8 +203,7 @@ class OSSStorageService:
             return signed
         except Exception:
             logger.exception(
-                "get_download_url: sign_url failed, falling back to raw URL. "
-                "storage_key=%s",
+                "get_download_url: sign_url failed, falling back to raw URL. " "storage_key=%s",
                 storage_key[:200],
             )
             return self.get_url(storage_key)
@@ -256,4 +274,5 @@ def get_storage_service() -> OSSStorageService:
     global _storage_service
     if _storage_service is None:
         _storage_service = OSSStorageService()
+        _storage_service.diagnose()
     return _storage_service