fix: prevent race conditions during vault index rebuild and rate-limit embeddings

- Add AbortController to cancel ongoing indexing before clear/rebuild
- Track currentIndexingPromise to await cancellation before clearing collection
- Batch background indexing (5 files at a time) with 100ms delays between batches
- Skip incremental event-based indexing while a full rebuild is in progress
- Cancel indexing on plugin unload
- Fix nginx CORS headers on OPTIONS preflight responses

nginx.conf

@@ -8,7 +8,7 @@ http {
         server_name localhost;
 
         location / {
-            # CORS headers for every response (including 4xx/5xx from upstream)
+            # CORS headers must be set for every response, including OPTIONS preflight
             add_header 'Access-Control-Allow-Origin' '*' always;
             add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
             add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range,Authorization' always;
@@ -16,6 +16,9 @@ http {
 
             # Preflight OPTIONS
             if ($request_method = 'OPTIONS') {
+                add_header 'Access-Control-Allow-Origin' '*' always;
+                add_header 'Access-Control-Allow-Methods' 'GET, POST, PUT, DELETE, OPTIONS' always;
+                add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range,Authorization' always;
                 add_header 'Access-Control-Max-Age' 1728000;
                 add_header 'Content-Type' 'text/plain; charset=utf-8';
                 add_header 'Content-Length' 0;