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.

cista/util/diskspace.py

@@ -0,0 +1,49 @@
+import shutil
+import threading
+import time
+from pathlib import Path
+
+MIN_FREE_BYTES = 128 * 1024 * 1024
+_CHECK_CACHE_TTL = 1.0
+
+
+class InsufficientStorageError(Exception):
+    """Raised when there is not enough disk space for an operation."""
+
+
+_cache: dict[Path, tuple[float, int]] = {}
+_lock = threading.Lock()
+
+
+def check_free_space(path: Path) -> None:
+    """Raise InsufficientStorageError if free space on the filesystem containing *path*
+
+    is below MIN_FREE_BYTES. Results are cached per directory for 1 second.
+    """
+    check_path = path.parent if path.parent.exists() else path
+    check_path = check_path.resolve()
+
+    now = time.monotonic()
+    with _lock:
+        ts, free = _cache.get(check_path, (0, 0))
+        if now - ts < _CHECK_CACHE_TTL:
+            if free < MIN_FREE_BYTES:
+                raise InsufficientStorageError(
+                    f"Insufficient storage: {free} bytes free, "
+                    f"need at least {MIN_FREE_BYTES} bytes"
+                )
+            return
+
+    try:
+        free = shutil.disk_usage(check_path).free
+    except OSError as e:
+        raise InsufficientStorageError(f"Cannot check disk usage: {e}") from e
+
+    with _lock:
+        _cache[check_path] = (now, free)
+
+    if free < MIN_FREE_BYTES:
+        raise InsufficientStorageError(
+            f"Insufficient storage: {free} bytes free, "
+            f"need at least {MIN_FREE_BYTES} bytes"
+        )