Stop the chunker looping forever on blank documents

A document whose text is only whitespace produces no chunks, so it was
selected again on every pass and enrich.embed never finished.

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

backend/politrack/enrich/embed.py

@@ -78,12 +78,12 @@ def build_chunks(conn: Connection, batch: int = 5000) -> int:
     docs = conn.execute(
         """
         SELECT d.id, d.title, d.full_text FROM document d
-        WHERE d.full_text IS NOT NULL AND NOT EXISTS (SELECT 1 FROM chunk ch WHERE ch.document_id = d.id)
+        WHERE d.full_text ~ '\S' AND NOT EXISTS (SELECT 1 FROM chunk ch WHERE ch.document_id = d.id)
         LIMIT %s
         """,
         (batch // 10,),
     ).fetchall()
-    for d in docs:
+    for d in docs:  # blank texts are excluded above: they yield no chunks and would be selected forever
         for i, part in enumerate(split_text(d["full_text"])):
             conn.execute(UPSERT_CHUNK, (None, d["id"], i, f"{d['title'] or ''}\n{part}".strip()))
     conn.commit()