fix: use collection deletion instead of reset, add embedding retry, slow down indexing

- semantic-cache.clearCache: delete collection instead of client.reset()
  to avoid 403 Forbidden on newer ChromaDB versions
- vault-vector-store.clearIndex: same collection deletion approach
- ContentVectorizer: add 3-attempt retry with exponential backoff (1s, 2s, 4s)
- main.ts: reduce indexing batch size from 5 to 2, increase delay from 100ms to 500ms
- Update semantic-cache tests for deleteCollection

tests/semantic-cache.test.ts

@@ -155,13 +155,15 @@ describe('SemanticCacheService', () => {
 
       await disabledCacheService.clearCache();
 
-      expect(mockChromaClient.reset).not.toHaveBeenCalled();
+      expect(mockChromaClient.deleteCollection).not.toHaveBeenCalled();
     });
 
-    it('should clear the cache via the ChromaClient', async () => {
+    it('should clear the cache via deleteCollection', async () => {
       await cacheService.clearCache();
 
-      expect(mockChromaClient.reset).toHaveBeenCalled();
+      expect(mockChromaClient.deleteCollection).toHaveBeenCalledWith({
+        name: mockCacheConfig.collectionName,
+      });
     });
   });
 });