feat(attachments): sha-addressed AttachmentStore

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_attachment_store.py

@@ -0,0 +1,28 @@
+from pathlib import Path
+
+from backend.app.services.attachment_store import AttachmentStore
+
+
+def test_store_layout_and_dedup(tmp_path):
+    src = tmp_path / "pack.zip"
+    src.write_bytes(b"\x00\x01binary\xff")
+    store = AttachmentStore(images_root=tmp_path / "images")
+    sha = "ab" + "c" * 62
+
+    p1 = store.store(src, sha)
+    assert p1.endswith(f"attachments/{sha[:3]}/{sha}.zip")
+    assert Path(p1).read_bytes() == b"\x00\x01binary\xff"
+
+    # second store of same sha: returns same path, no rewrite
+    before = Path(p1).stat().st_mtime_ns
+    p2 = store.store(src, sha)
+    assert p2 == p1
+    assert Path(p1).stat().st_mtime_ns == before
+
+
+def test_store_preserves_extension_case_insensitive(tmp_path):
+    src = tmp_path / "Doc.PDF"
+    src.write_bytes(b"%PDF-1.4")
+    store = AttachmentStore(images_root=tmp_path / "images")
+    p = store.store(src, "f" * 64)
+    assert p.endswith(".pdf")