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/urllib3/util/util.py

@@ -0,0 +1,42 @@
+from __future__ import annotations
+
+import typing
+from types import TracebackType
+
+
+def to_bytes(
+    x: str | bytes, encoding: str | None = None, errors: str | None = None
+) -> bytes:
+    if isinstance(x, bytes):
+        return x
+    elif not isinstance(x, str):
+        raise TypeError(f"not expecting type {type(x).__name__}")
+    if encoding or errors:
+        return x.encode(encoding or "utf-8", errors=errors or "strict")
+    return x.encode()
+
+
+def to_str(
+    x: str | bytes, encoding: str | None = None, errors: str | None = None
+) -> str:
+    if isinstance(x, str):
+        return x
+    elif not isinstance(x, bytes):
+        raise TypeError(f"not expecting type {type(x).__name__}")
+    if encoding or errors:
+        return x.decode(encoding or "utf-8", errors=errors or "strict")
+    return x.decode()
+
+
+def reraise(
+    tp: type[BaseException] | None,
+    value: BaseException,
+    tb: TracebackType | None = None,
+) -> typing.NoReturn:
+    try:
+        if value.__traceback__ is not tb:
+            raise value.with_traceback(tb)
+        raise value
+    finally:
+        value = None  # type: ignore[assignment]
+        tb = None