feat: add plaintext parser to ETL pipeline for reading text files

surfsense_backend/app/etl_pipeline/parsers/plaintext.py

@@ -0,0 +1,8 @@
+def read_plaintext(file_path: str) -> str:
+    with open(file_path, encoding="utf-8", errors="replace") as f:
+        content = f.read()
+    if "\x00" in content:
+        raise ValueError(
+            f"File contains null bytes — likely a binary file opened as text: {file_path}"
+        )
+    return content