Refactor: extract shared image upload utilities into bincio/shared/images.py

ALLOWED_IMAGE_TYPES, MAX_IMAGE_BYTES, and unique_image_name() were
duplicated identically in both the edit and serve servers. Centralising
them means a single change point for any future extension (e.g. adding
image/avif support).

Tests added in tests/test_shared_images.py cover no-collision, single
and chained collisions, no-suffix filenames, and constant values.

bincio/shared/images.py

@@ -0,0 +1,23 @@
+"""Shared image upload utilities used by both the edit and serve servers."""
+from __future__ import annotations
+
+from pathlib import Path
+
+ALLOWED_IMAGE_TYPES: frozenset[str] = frozenset({
+    "image/jpeg",
+    "image/png",
+    "image/webp",
+    "image/gif",
+})
+MAX_IMAGE_BYTES: int = 10 * 1024 * 1024  # 10 MB
+
+
+def unique_image_name(directory: Path, filename: str) -> str:
+    """Return a filename that does not collide with existing files in directory."""
+    stem, suffix = Path(filename).stem, Path(filename).suffix
+    candidate = filename
+    counter = 1
+    while (directory / candidate).exists():
+        candidate = f"{stem}_{counter}{suffix}"
+        counter += 1
+    return candidate