Fix numpy-free metadata mode after the pixel-layer merge

Guard module-level numpy touchpoints so the metadata-only mode imports
and runs without numpy installed (deferred annotations, lazy DCT basis);
pixel modes now exit early with a clear numpy requirement message.

scripts/scan_dataset.py

@@ -1,5 +1,4 @@
 """Collect all raw metadata over a dataset, for later offline analysis.
-
 Read-only; analysis is NOT this script's job. For every image it writes
 one JSONL record containing:
 
@@ -55,6 +54,8 @@ Dependencies: pip install pillow piexif c2pa-python pillow-heif
 (pillow-heif is only needed for HEIC/AVIF inputs).
 """
 
+from __future__ import annotations
+
 import base64
 import contextlib
 import csv
@@ -954,7 +955,7 @@ def _dct_matrix(n: int = 8) -> np.ndarray:
     return m * np.sqrt(2 / n)
 
 
-_DCT_M = _dct_matrix()
+_DCT_M = _dct_matrix() if np is not None else None
 
 
 def dct_features(gray: np.ndarray) -> dict[str, Any]: