Add targeted comments explaining non-obvious behaviour

- embedder.py: lazy model load rationale, RGB conversion, shared vector space
- main.py: why vec appears twice, ::vector cast, 1-distance score formula
- main_oracle.py: why array.array("f") is required instead of plain list
- main_oracle_indb.py: no embedder import — embedding done inside Oracle SQL
- index_images_oracle.py: same array.array requirement on indexing path

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

pgvector-demo/backend/embedder.py

@@ -4,14 +4,19 @@ from PIL import Image
 _model = None
 
 def _get_model():
+    # Lazy load: the CLIP model is ~600 MB and takes several seconds to initialise.
+    # Loading on first call avoids the cost at import time and during indexing warmup.
     global _model
     if _model is None:
         _model = SentenceTransformer("clip-ViT-B-32")
     return _model
 
 def embed_image(path: str) -> list[float]:
+    # CLIP requires RGB — some JPEGs are stored as CMYK or grayscale.
     img = Image.open(path).convert("RGB")
     return _get_model().encode(img).tolist()
 
 def embed_text(text: str) -> list[float]:
+    # Text and images share the same 512-dimensional vector space in CLIP,
+    # so the returned vector is directly comparable to image embeddings.
     return _get_model().encode(text).tolist()