feat(attachments): sha-addressed AttachmentStore

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

backend/app/services/attachment_store.py

@@ -0,0 +1,27 @@
+"""sha-addressed store for preserved non-art post files.
+
+Arbitrary binaries — never opened or validated as images. Parallels the
+thumbnail store layout (/images/attachments/<sha[:3]>/<sha><ext>).
+"""
+
+import shutil
+from pathlib import Path
+
+
+class AttachmentStore:
+    def __init__(self, images_root: Path):
+        self.root = Path(images_root) / "attachments"
+
+    def store(self, src: Path, sha256: str) -> str:
+        """Copy src into the sha-addressed store; idempotent on sha.
+        Returns the stored absolute path as a string."""
+        ext = Path(src).suffix.lower()
+        dest_dir = self.root / sha256[:3]
+        dest_dir.mkdir(parents=True, exist_ok=True)
+        dest = dest_dir / f"{sha256}{ext}"
+        if dest.exists():
+            return str(dest)
+        partial = dest.with_suffix(dest.suffix + ".partial")
+        shutil.copy2(src, partial)
+        partial.rename(dest)
+        return str(dest)

tests/test_attachment_store.py

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+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")