feat: implement ETL pipeline with file classification and extraction services

surfsense_backend/app/etl_pipeline/etl_document.py

@@ -0,0 +1,21 @@
+from pydantic import BaseModel, field_validator
+
+
+class EtlRequest(BaseModel):
+    file_path: str
+    filename: str
+    estimated_pages: int = 0
+
+    @field_validator("filename")
+    @classmethod
+    def filename_must_not_be_empty(cls, v: str) -> str:
+        if not v.strip():
+            raise ValueError("filename must not be empty")
+        return v
+
+
+class EtlResult(BaseModel):
+    markdown_content: str
+    etl_service: str
+    actual_pages: int = 0
+    content_type: str