Enforce 128 MiB minimum free space on uploads

- Add cista/util/diskspace.py with MIN_FREE_BYTES limit and cached
  check_free_space() helper.
- Check available space in File.write() before ftruncate/write.
- Catch ENOSPC in File.write and re-raise as InsufficientStorageError.
- upload_file_chunk catches both proactive and ENOSPC errors and
  returns HTTP 507 Insufficient Storage.
- Add tests for low-disk rejection and ENOSPC handling.

tests/test_disk_space.py

@@ -0,0 +1,65 @@
+import errno
+from pathlib import Path
+from typing import NamedTuple
+from unittest.mock import patch
+from uuid import uuid4
+
+import pytest
+import pytest_asyncio
+from sanic import Sanic
+
+from cista import config, watching
+from cista.api import fileserver
+from cista.fileserver import bp as fileserver_bp
+
+
+class Usage(NamedTuple):
+    total: int
+    used: int
+    free: int
+
+
+def _low_disk_usage(*args, **kwargs):
+    return Usage(total=1000, used=900, free=10)
+
+
+@pytest.fixture
+def setup_storage(tmp_path: Path):
+    config.config = config.Config(path=tmp_path, listen=":0", public=True)
+    watching.state.root = []
+    watching.rootpath = tmp_path
+    yield tmp_path
+    watching.state.root = []
+
+
+@pytest_asyncio.fixture()
+async def client(setup_storage: Path):
+    app = Sanic(f"disk-space-test-{uuid4().hex}", strict_slashes=True)
+    app.router.ALLOWED_METHODS = (
+        *app.router.ALLOWED_METHODS,
+        "MKCOL",
+        "MOVE",
+        "COPY",
+        "PROPFIND",
+    )
+    app.blueprint(fileserver_bp)
+    await fileserver.start()
+    yield app.asgi_client
+    await fileserver.stop()
+
+
+@pytest.mark.asyncio
+async def test_upload_rejected_when_disk_low(client):
+    with patch("cista.util.diskspace.shutil.disk_usage", side_effect=_low_disk_usage):
+        _, res = await client.put("/files/test.txt", data=b"hello world")
+    assert res.status_code == 507
+
+
+@pytest.mark.asyncio
+async def test_upload_rejected_on_enospc(client):
+    with patch(
+        "cista.fileio.os.write",
+        side_effect=OSError(errno.ENOSPC, "No space left on device"),
+    ):
+        _, res = await client.put("/files/test.txt", data=b"hello world")
+    assert res.status_code == 507