Use a raw string for the chunker's regex query

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/politrack/enrich/embed.py

@@ -76,7 +76,7 @@ def build_chunks(conn: Connection, batch: int = 5000) -> int:
         conn.execute(UPSERT_CHUNK, (c["id"], None, 0, text.strip()))
 
     docs = conn.execute(
-        """
+        r"""
         SELECT d.id, d.title, d.full_text FROM document d
         WHERE d.full_text ~ '\S' AND NOT EXISTS (SELECT 1 FROM chunk ch WHERE ch.document_id = d.id)
         LIMIT %s