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/starlette/middleware/authentication.py

@@ -0,0 +1,52 @@
+from __future__ import annotations
+
+from collections.abc import Callable
+
+from starlette.authentication import (
+    AuthCredentials,
+    AuthenticationBackend,
+    AuthenticationError,
+    UnauthenticatedUser,
+)
+from starlette.requests import HTTPConnection
+from starlette.responses import PlainTextResponse, Response
+from starlette.types import ASGIApp, Receive, Scope, Send
+
+
+class AuthenticationMiddleware:
+    def __init__(
+        self,
+        app: ASGIApp,
+        backend: AuthenticationBackend,
+        on_error: Callable[[HTTPConnection, AuthenticationError], Response] | None = None,
+    ) -> None:
+        self.app = app
+        self.backend = backend
+        self.on_error: Callable[[HTTPConnection, AuthenticationError], Response] = (
+            on_error if on_error is not None else self.default_on_error
+        )
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        if scope["type"] not in ["http", "websocket"]:
+            await self.app(scope, receive, send)
+            return
+
+        conn = HTTPConnection(scope)
+        try:
+            auth_result = await self.backend.authenticate(conn)
+        except AuthenticationError as exc:
+            response = self.on_error(conn, exc)
+            if scope["type"] == "websocket":
+                await send({"type": "websocket.close", "code": 1000})
+            else:
+                await response(scope, receive, send)
+            return
+
+        if auth_result is None:
+            auth_result = AuthCredentials(), UnauthenticatedUser()
+        scope["auth"], scope["user"] = auth_result
+        await self.app(scope, receive, send)
+
+    @staticmethod
+    def default_on_error(conn: HTTPConnection, exc: Exception) -> Response:
+        return PlainTextResponse(str(exc), status_code=400)