Add in-DB indexing script, benchmark results, schema names in presentation

- index_images_indb.py: new script indexing via VECTOR_EMBEDDING(CLIP_IMG)
  using a two-step INSERT+UPDATE to work around ORA-24816
- index_images_oracle.py / index_images.py: add timing output
- Presentation: schema names VECTORS_USER/VECTOR in diagram and comparison,
  ONNX expansion, HNSW index note on slide 11,
  indexing times updated from 3-run benchmark (avg: PG 12.1s, Ora 12.1s, InDB 13.6s)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pgvector-demo/backend/index_images.py

@@ -1,4 +1,5 @@
 import os
+import time
 from dotenv import load_dotenv
 from db import get_connection
 from embedder import embed_image
@@ -37,6 +38,7 @@ def main():
     files = [f for f in os.listdir(PHOTOS_DIR) if f.lower().endswith((".jpg", ".jpeg"))]
     print(f"Found {len(files)} photos in {PHOTOS_DIR}")
 
+    start = time.time()
     for i, filename in enumerate(files, 1):
         filepath = os.path.join(PHOTOS_DIR, filename)
         cur.execute("SELECT 1 FROM images WHERE filename = %s", (filename,))
@@ -50,7 +52,7 @@ def main():
 
     cur.close()
     conn.close()
-    print("Done.")
+    print(f"Done in {time.time() - start:.1f} seconds.")
 
 if __name__ == "__main__":
     main()