Perf: count compact()'s live rows via document_chunks, not a vec0 scan

compact() ran SELECT count(*) on the vec0 table itself purely to compute
the bloat ratio, on every update_llm_index() call -- including small
scoped ones from bulk_update_documents. Like any other document_id-scale
lookup, count(*) has no point-lookup fast path on vec0's own table, so
this is a full scan regardless of index size.

document_chunks is kept in lockstep with the vec0 table by construction
and is a plain indexed table, so it gives an identical count far more
cheaply. Only affects the bloat-ratio heuristic and a log line -- the
actual rebuild already streams and counts real vec0 rows independently
(_copy_rows()'s own return value), so this can't desync compaction's
correctness even in the brief window before a store's document_chunks is
fully backfilled by migration.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/paperless_ai/vector_store.py

@@ -599,9 +599,12 @@ class PaperlessSqliteVecVectorStore(BasePydanticVectorStore):
         """
         if not self.table_exists():
             return
-        live = self._conn.execute(
-            "SELECT count(*) FROM " + DEFAULT_TABLE_NAME,
-        ).fetchone()[0]
+        # document_chunks is a plain indexed table kept in lockstep with the
+        # vec0 table (see _index_chunks()/_delete_chunks_by_document_id()),
+        # so it gives an identical live-row count to `SELECT count(*) FROM
+        # documents` without vec0's full table scan -- count(*) has no
+        # equivalent point-lookup fast path, unlike an EQ constraint on `id`.
+        live = self._conn.execute("SELECT count(*) FROM document_chunks").fetchone()[0]
         total = int(self._meta_get("total_inserts") or str(live))
         if not force and total <= max(live, 1) * COMPACT_BLOAT_RATIO:
             return