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/PaletteFile.py

@@ -0,0 +1,54 @@
+#
+# Python Imaging Library
+# $Id$
+#
+# stuff to read simple, teragon-style palette files
+#
+# History:
+#       97-08-23 fl     Created
+#
+# 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 typing import IO
+
+from ._binary import o8
+
+
+class PaletteFile:
+    """File handler for Teragon-style palette files."""
+
+    rawmode = "RGB"
+
+    def __init__(self, fp: IO[bytes]) -> None:
+        palette = [o8(i) * 3 for i in range(256)]
+
+        while True:
+            s = fp.readline()
+
+            if not s:
+                break
+            if s.startswith(b"#"):
+                continue
+            if len(s) > 100:
+                msg = "bad palette file"
+                raise SyntaxError(msg)
+
+            v = [int(x) for x in s.split()]
+            try:
+                [i, r, g, b] = v
+            except ValueError:
+                [i, r] = v
+                g = b = r
+
+            if 0 <= i <= 255:
+                palette[i] = o8(r) + o8(g) + o8(b)
+
+        self.palette = b"".join(palette)
+
+    def getpalette(self) -> tuple[bytes, str]:
+        return self.palette, self.rawmode