fix: synchronize SQLite cache for virtual thread safety

AnalysisCache.storeResults() called from multiple virtual threads
concurrently caused "cannot commit - no transaction is active"
errors. Fixed by synchronizing on the connection object.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: aksOps

src/main/java/io/github/randomcodespace/iq/cache/AnalysisCache.java

@@ -158,6 +158,7 @@ public boolean isCached(String contentHash) {
      */
     public void storeResults(String contentHash, String filePath, String language,
                              List<CodeNode> nodes, List<CodeEdge> edges) {
+        synchronized (conn) {
         try {
             conn.setAutoCommit(false);
             String now = Instant.now().toString();
@@ -222,6 +223,7 @@ public void storeResults(String contentHash, String filePath, String language,
             } catch (SQLException ignored) {
             }
         }
+        } // synchronized
     }
 
     // --- Load cached results ---