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/sniffio/_impl.py

@@ -0,0 +1,95 @@
+from contextvars import ContextVar
+from typing import Optional
+import sys
+import threading
+
+current_async_library_cvar = ContextVar(
+    "current_async_library_cvar", default=None
+)  # type: ContextVar[Optional[str]]
+
+
+class _ThreadLocal(threading.local):
+    # Since threading.local provides no explicit mechanism is for setting
+    # a default for a value, a custom class with a class attribute is used
+    # instead.
+    name = None  # type: Optional[str]
+
+
+thread_local = _ThreadLocal()
+
+
+class AsyncLibraryNotFoundError(RuntimeError):
+    pass
+
+
+def current_async_library() -> str:
+    """Detect which async library is currently running.
+
+    The following libraries are currently supported:
+
+    ================   ===========  ============================
+    Library             Requires     Magic string
+    ================   ===========  ============================
+    **Trio**            Trio v0.6+   ``"trio"``
+    **Curio**           -            ``"curio"``
+    **asyncio**                      ``"asyncio"``
+    **Trio-asyncio**    v0.8.2+     ``"trio"`` or ``"asyncio"``,
+                                    depending on current mode
+    ================   ===========  ============================
+
+    Returns:
+      A string like ``"trio"``.
+
+    Raises:
+      AsyncLibraryNotFoundError: if called from synchronous context,
+        or if the current async library was not recognized.
+
+    Examples:
+
+        .. code-block:: python3
+
+           from sniffio import current_async_library
+
+           async def generic_sleep(seconds):
+               library = current_async_library()
+               if library == "trio":
+                   import trio
+                   await trio.sleep(seconds)
+               elif library == "asyncio":
+                   import asyncio
+                   await asyncio.sleep(seconds)
+               # ... and so on ...
+               else:
+                   raise RuntimeError(f"Unsupported library {library!r}")
+
+    """
+    value = thread_local.name
+    if value is not None:
+        return value
+
+    value = current_async_library_cvar.get()
+    if value is not None:
+        return value
+
+    # Need to sniff for asyncio
+    if "asyncio" in sys.modules:
+        import asyncio
+        try:
+            current_task = asyncio.current_task  # type: ignore[attr-defined]
+        except AttributeError:
+            current_task = asyncio.Task.current_task  # type: ignore[attr-defined]
+        try:
+            if current_task() is not None:
+                return "asyncio"
+        except RuntimeError:
+            pass
+
+    # Sniff for curio (for now)
+    if 'curio' in sys.modules:
+        from curio.meta import curio_running
+        if curio_running():
+            return 'curio'
+
+    raise AsyncLibraryNotFoundError(
+        "unknown async library, or not in async context"
+    )