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/pluggy/_warnings.py

@@ -0,0 +1,27 @@
+from typing import final
+
+
+class PluggyWarning(UserWarning):
+    """Base class for all warnings emitted by pluggy."""
+
+    __module__ = "pluggy"
+
+
+@final
+class PluggyTeardownRaisedWarning(PluggyWarning):
+    """A plugin raised an exception during an :ref:`old-style hookwrapper
+    <old_style_hookwrappers>` teardown.
+
+    Such exceptions are not handled by pluggy, and may cause subsequent
+    teardowns to be executed at unexpected times, or be skipped entirely.
+
+    This is an issue in the plugin implementation.
+
+    If the exception is unintended, fix the underlying cause.
+
+    If the exception is intended, switch to :ref:`new-style hook wrappers
+    <hookwrappers>`, or use :func:`result.force_exception()
+    <pluggy.Result.force_exception>` to set the exception instead of raising.
+    """
+
+    __module__ = "pluggy"