fix(storage): #106.5 — serve_blob probes S3 sub-backend, not dual disk-OR-S3 exists

באג-אינטראקציה שהתגלה לפני ה-flip ל-dual: DualBackend.exists() מחזיר True אם הקובץ על
**הדיסק או** ב-S3. serve_blob בדק backend.exists() ואז הנפיק presigned — כך שתחת dual,
קובץ שקיים-רק-בדיסק (mirror שנכשל / מחוץ לסט-ההגירה) היה מקבל redirect ל-presigned-URL
שמחזיר 404 מ-MinIO, במקום fallback-לדיסק.

תיקון: serve_blob בודק קיום ב-**S3 ספציפית** — `s3 = getattr(backend, "s3", backend)`
(DualBackend.s3, או ה-S3Backend עצמו תחת s3) — כך שקובץ disk-only נופל ל-FileResponse
אמיתי. תואם-לאחור ל-filesystem/s3 (getattr מחזיר את ה-backend עצמו).

invariants: INV-STG6 (presigned רק כשהאובייקט באמת ב-S3) · INV-G10 (אפס שינוי תחת filesystem).
tests: 6 (2 חדשות — dual מ-S3-sub-backend present→redirect / absent→disk-fallback). py_compile OK.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/app.py

@@ -2789,14 +2789,21 @@ async def serve_blob(
     from pathlib import Path as _Path
     backend = storage.get_storage()
     if backend.name != "filesystem":
+        # Presign ONLY when the object is actually in S3. Under ``dual`` the
+        # generic ``exists`` is disk-OR-S3, so a disk-only file (failed mirror /
+        # outside the migration set) would otherwise be redirected to a presigned
+        # URL that 404s on MinIO. Probe the S3 sub-backend specifically
+        # (DualBackend.s3 — or the S3 backend itself under ``s3``) so disk-only
+        # files fall back to a real disk read.
+        s3 = getattr(backend, "s3", backend)
         try:
             key = storage.normalize_key(path)
-            if await backend.exists(key, bucket=bucket):
-                url = await backend.presign_get(key, bucket=bucket, download_name=filename)
+            if await s3.exists(key, bucket=bucket):
+                url = await s3.presign_get(key, bucket=bucket, download_name=filename)
                 return RedirectResponse(url, status_code=302)
         except Exception:  # noqa: BLE001 — never 500 on a key/presign hiccup; fall back to disk
             logger.exception("serve_blob: presign failed for %s; falling back to disk", path)
-        # not in object storage (dual fallback) — serve the disk copy if present
+        # not in object storage (dual disk-fallback) — serve the disk copy if present
         if not _Path(path).exists():
             raise HTTPException(404, "קובץ לא נמצא")
     return FileResponse(path, media_type=media_type, filename=filename)

web/tests/test_serve_blob.py

@@ -78,3 +78,34 @@ def test_s3_missing_no_disk_404(monkeypatch):
     with pytest.raises(HTTPException) as ei:
         _serve(monkeypatch, "s3", False, DATA_DIR / "audit" / "_nope.txt")
     assert ei.value.status_code == 404
+
+
+class _FakeDual:
+    """DualBackend stand-in: generic exists() is disk-OR-S3 (always True here),
+    but the .s3 sub-backend is S3-only. The fix must probe .s3, not exists()."""
+    name = "dual"
+
+    def __init__(self, s3_has: bool) -> None:
+        self.s3 = _FakeBackend("s3", s3_has)
+
+    async def exists(self, key, *, bucket) -> bool:  # noqa: ANN001 — disk-or-S3 → True
+        return True
+
+    async def presign_get(self, key, *, bucket, download_name=None) -> str:  # noqa: ANN001
+        return f"https://s3.example/WRONG/{key}"  # must NOT be used (proves .s3 probed)
+
+
+def test_dual_probes_s3_subbackend_not_generic_exists(blob, monkeypatch):
+    """Regression (DualBackend.exists is disk-OR-S3): a file on disk but NOT in
+    S3 must fall back to a disk FileResponse, never a presigned URL that 404s."""
+    monkeypatch.setattr(storage, "get_storage", lambda: _FakeDual(s3_has=False))
+    r = asyncio.new_event_loop().run_until_complete(
+        app.serve_blob(str(blob), media_type="text/plain", filename="x.txt"))
+    assert isinstance(r, FileResponse)  # disk fallback, NOT a (broken) redirect
+
+
+def test_dual_in_s3_uses_s3_subbackend(blob, monkeypatch):
+    monkeypatch.setattr(storage, "get_storage", lambda: _FakeDual(s3_has=True))
+    r = asyncio.new_event_loop().run_until_complete(
+        app.serve_blob(str(blob), media_type="text/plain", filename="x.txt"))
+    assert isinstance(r, RedirectResponse)