Revert "fix: switch embedding dimension from 1536 to 2560 for qwen-embedding-4b"

This reverts commit e09ea3a145.

memento-note/prisma/migrations/20260512120000_pgvector_and_fts_search/migration.sql

@@ -1,7 +1,7 @@
 -- Phase 1: Enable pgvector extension
 CREATE EXTENSION IF NOT EXISTS vector;
 
--- Phase 2: Convert embedding column from text to vector(2560)
+-- Phase 2: Convert embedding column from text to vector(1536)
 -- Idempotent: detects current column type and only converts when needed.
 -- Handles all partial states from previous failed migration attempts:
 --   A) embedding is text → direct ALTER COLUMN TYPE conversion
@@ -25,8 +25,8 @@ BEGIN
     IF _emb_type IS NOT NULL THEN
         ALTER TABLE "NoteEmbedding" DROP COLUMN IF EXISTS "_vec_tmp";
         ALTER TABLE "NoteEmbedding"
-            ALTER COLUMN "embedding" TYPE vector(2560)
-            USING "embedding"::vector(2560);
+            ALTER COLUMN "embedding" TYPE vector(1536)
+            USING "embedding"::vector(1536);
         RETURN;
     END IF;