feat(crops): shared crop primitive for the region/crop pipeline (#114)

The trunk of both crop jobs — CCIP figure-crops and SigLIP concept-crops call
the SAME crop_region(): normalized-bbox crop with optional context padding,
edge-clamping, and the lower-bound size floor (max of a fraction-of-short-side
and an absolute pixel floor) below which a region is too small to embed and
returns None. Only the proposer (where) and embedder (what) differ; the crop is
shared. Pure Pillow — importable + testable anywhere (the GPU agent imports it
for the crop step). Unit-lane tests (no DB): region pixels, floor rejection,
edge clamp, pad expansion, out-size resize.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

tests/test_crops.py

@@ -0,0 +1,44 @@
+"""Shared crop primitive (#114) — pure Pillow, no DB, so it runs in the fast
+unit lane (no integration marker)."""
+from PIL import Image
+
+from backend.app.services.ml.crops import crop_region
+
+
+def _quadrant_img():
+    """400x400 red with a blue bottom-right quadrant, so a crop's content is
+    checkable by pixel."""
+    img = Image.new("RGB", (400, 400), (255, 0, 0))
+    img.paste(Image.new("RGB", (200, 200), (0, 0, 255)), (200, 200))
+    return img
+
+
+def test_crop_returns_region_pixels():
+    crop = crop_region(_quadrant_img(), (0.5, 0.5, 0.5, 0.5))
+    assert crop is not None
+    assert crop.size == (200, 200)
+    assert crop.getpixel((100, 100)) == (0, 0, 255)   # the blue quadrant
+
+
+def test_crop_below_floor_is_rejected():
+    # 0.05 * 400 = 20px on a side — below max(64, 0.10*400=40) → None.
+    assert crop_region(_quadrant_img(), (0.0, 0.0, 0.05, 0.05)) is None
+
+
+def test_crop_clamped_to_image_bounds():
+    # Box runs off the right/bottom edge; clamps to the remaining 0.2*400=80px.
+    crop = crop_region(_quadrant_img(), (0.8, 0.8, 0.5, 0.5))
+    assert crop is not None
+    assert crop.size == (80, 80)
+
+
+def test_pad_expands_the_crop():
+    base = crop_region(_quadrant_img(), (0.4, 0.4, 0.2, 0.2))
+    padded = crop_region(_quadrant_img(), (0.4, 0.4, 0.2, 0.2), pad=0.5)
+    assert base.size == (80, 80)
+    assert padded.size[0] > base.size[0] and padded.size[1] > base.size[1]
+
+
+def test_out_size_resizes_square():
+    crop = crop_region(_quadrant_img(), (0.25, 0.25, 0.5, 0.5), out_size=224)
+    assert crop.size == (224, 224)