feat(phase1): add image storage utilities

- Create backend/services/image_storage.py with 4 core functions:
  - sanitize_filename(): remove unsafe chars, limit to 255 chars, convert to lowercase
  - get_unique_filename(): handle collisions with UUID suffix (format: {name}_{uuid8}_{variant}.jpg)
  - ensure_image_directories(): create /images/ root and category subdirs on startup
  - save_image(): save bytes to /images/{category}/{filename}, returns relative path
- Create comprehensive test suite (22 tests) covering all functionality
- Integrate ensure_image_directories() into FastAPI startup event
- Directory structure: /images/{category}/{filename}
- Collision handling: auto-suffix with UUID if filename exists
- All tests passing, pathlib.Path for safe operations

venv/lib/python3.12/site-packages/click/_utils.py

@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+import enum
+import typing as t
+
+
+class Sentinel(enum.Enum):
+    """Enum used to define sentinel values.
+
+    .. seealso::
+
+        `PEP 661 - Sentinel Values <https://peps.python.org/pep-0661/>`_.
+    """
+
+    UNSET = object()
+    FLAG_NEEDS_VALUE = object()
+
+    def __repr__(self) -> str:
+        return f"{self.__class__.__name__}.{self.name}"
+
+
+UNSET = Sentinel.UNSET
+"""Sentinel used to indicate that a value is not set."""
+
+FLAG_NEEDS_VALUE = Sentinel.FLAG_NEEDS_VALUE
+"""Sentinel used to indicate an option was passed as a flag without a
+value but is not a flag option.
+
+``Option.consume_value`` uses this to prompt or use the ``flag_value``.
+"""
+
+T_UNSET = t.Literal[UNSET]  # type: ignore[valid-type]
+"""Type hint for the :data:`UNSET` sentinel value."""
+
+T_FLAG_NEEDS_VALUE = t.Literal[FLAG_NEEDS_VALUE]  # type: ignore[valid-type]
+"""Type hint for the :data:`FLAG_NEEDS_VALUE` sentinel value."""