Performance: use small vec0 chunks and larger pages for the LLM index (#14678)

The vec0 default of 1024 rows per vector chunk makes each chunk a
multi-megabyte blob. On spinning disks, indexing a document took about
2.5 seconds and re-indexing one about 13 seconds. With 32 rows per
chunk the same operations take about 35 and 180 milliseconds, with the
write-ahead log a third of the size and no cost in query speed or file
size.

Pair that with 64 KiB SQLite pages, which made cold scans about 35%
faster and warm ones about 45% faster at no write cost once the chunks
are small. The page size is set before the file enters WAL mode, so it
applies to new files.

src/paperless_ai/vector_store.py

@@ -47,6 +47,20 @@ SCHEMA_VERSION = 2
 # a rebuild copies the live rows into a fresh table.
 COMPACT_BLOAT_RATIO = 2.0
 
+# Rows per vec0 vector chunk (vec0's default is 1024, which makes each chunk a
+# multi-megabyte blob). On spinning disks, inserts and re-indexing were dozens
+# of times faster with small chunks, at no cost in query speed or file size.
+# Only applies to tables created from here on; an existing index picks it up
+# the next time it is rebuilt (compact or rebuild).
+VEC0_CHUNK_SIZE = 32
+
+# SQLite page size for the index file, in bytes. With the small vec0 chunks above,
+# 64 KiB pages made cold scans about 35% faster and warm ones about 45% faster,
+# at the cost of a larger write-ahead log (up to about 70 MB). Only takes effect
+# on a newly created file, so like the chunk size it reaches an existing index
+# when it is rebuilt.
+DB_PAGE_SIZE = 65536
+
 # Number of rows fetched/copied per batch whenever this module streams rows
 # instead of materializing them all at once, keeping memory bounded regardless
 # of index size -- used by compact()'s rebuild, m0001_v1_to_v2's migration
@@ -222,6 +236,7 @@ class PaperlessSqliteVecVectorStore(BasePydanticVectorStore):
         conn.enable_load_extension(True)  # noqa: FBT003
         sqlite_vec.load(conn)
         conn.enable_load_extension(False)  # noqa: FBT003
+        conn.execute(f"PRAGMA page_size={DB_PAGE_SIZE}")
         conn.execute("PRAGMA journal_mode=WAL")
         conn.execute("PRAGMA synchronous=NORMAL")
         IndexMetaTable.create(conn)
@@ -329,7 +344,8 @@ class PaperlessSqliteVecVectorStore(BasePydanticVectorStore):
             + " +node_content TEXT,"
             + " embedding float["
             + str(int(dim))
-            + "] distance_metric=cosine"
+            + "] distance_metric=cosine, chunk_size="
+            + str(VEC0_CHUNK_SIZE)
             + ")",
         )