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

This reverts commit e09ea3a145.

memento-note/lib/ai/services/embedding.service.ts

@@ -1,7 +1,7 @@
 /**
  * Embedding Service
  * Generates vector embeddings for semantic search and similarity analysis.
- * Stores embeddings as native pgvector(2560) in PostgreSQL.
+ * Stores embeddings as native pgvector(1536) in PostgreSQL.
  */
 
 import { getAIProvider } from '../factory'
@@ -14,7 +14,7 @@ export interface EmbeddingResult {
 }
 
 export class EmbeddingService {
-  private readonly EMBEDDING_DIMENSION = 2560
+  private readonly EMBEDDING_DIMENSION = 1536
 
   async generateEmbedding(text: string): Promise<EmbeddingResult> {
     if (!text || text.trim().length === 0) {