feat: implement pbUpsert helper for streamlined database operations and update related functions

src/lib/learningService.js

@@ -1,5 +1,5 @@
 import * as db from './db';
-import { callLLM } from './llm';
+import { callLLM, cachedSystem } from './llm';
 import {
   EMIT_LEARNING_ARTICLE_TOOL,
   EMIT_LEARNING_SLIDES_TOOL,
@@ -17,8 +17,6 @@ Always write in clear, professional English.
 
 Emit the requested content through the matching tool — do not return prose JSON.`;
 
-const cachedSystem = (text) => [{ type: 'text', text, cache_control: { type: 'ephemeral' } }];
-
 const TOOL_BY_TYPE = {
   article: EMIT_LEARNING_ARTICLE_TOOL,
   slides: EMIT_LEARNING_SLIDES_TOOL,