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/PIL/PixarImagePlugin.py

@@ -0,0 +1,72 @@
+#
+# The Python Imaging Library.
+# $Id$
+#
+# PIXAR raster support for PIL
+#
+# history:
+#       97-01-29 fl     Created
+#
+# notes:
+#       This is incomplete; it is based on a few samples created with
+#       Photoshop 2.5 and 3.0, and a summary description provided by
+#       Greg Coats <gcoats@labiris.er.usgs.gov>.  Hopefully, "L" and
+#       "RGBA" support will be added in future versions.
+#
+# Copyright (c) Secret Labs AB 1997.
+# Copyright (c) Fredrik Lundh 1997.
+#
+# See the README file for information on usage and redistribution.
+#
+from __future__ import annotations
+
+from . import Image, ImageFile
+from ._binary import i16le as i16
+
+#
+# helpers
+
+
+def _accept(prefix: bytes) -> bool:
+    return prefix.startswith(b"\200\350\000\000")
+
+
+##
+# Image plugin for PIXAR raster images.
+
+
+class PixarImageFile(ImageFile.ImageFile):
+    format = "PIXAR"
+    format_description = "PIXAR raster image"
+
+    def _open(self) -> None:
+        # assuming a 4-byte magic label
+        assert self.fp is not None
+
+        s = self.fp.read(4)
+        if not _accept(s):
+            msg = "not a PIXAR file"
+            raise SyntaxError(msg)
+
+        # read rest of header
+        s = s + self.fp.read(508)
+
+        self._size = i16(s, 418), i16(s, 416)
+
+        # get channel/depth descriptions
+        mode = i16(s, 424), i16(s, 426)
+
+        if mode == (14, 2):
+            self._mode = "RGB"
+        # FIXME: to be continued...
+
+        # create tile descriptor (assuming "dumped")
+        self.tile = [ImageFile._Tile("raw", (0, 0) + self.size, 1024, self.mode)]
+
+
+#
+# --------------------------------------------------------------------
+
+Image.register_open(PixarImageFile.format, PixarImageFile, _accept)
+
+Image.register_extension(PixarImageFile.format, ".pxr")