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/fastapi/middleware/__init__.py

@@ -0,0 +1 @@
+from starlette.middleware import Middleware as Middleware

venv/lib/python3.12/site-packages/fastapi/middleware/asyncexitstack.py

@@ -0,0 +1,18 @@
+from contextlib import AsyncExitStack
+
+from starlette.types import ASGIApp, Receive, Scope, Send
+
+
+# Used mainly to close files after the request is done, dependencies are closed
+# in their own AsyncExitStack
+class AsyncExitStackMiddleware:
+    def __init__(
+        self, app: ASGIApp, context_name: str = "fastapi_middleware_astack"
+    ) -> None:
+        self.app = app
+        self.context_name = context_name
+
+    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
+        async with AsyncExitStack() as stack:
+            scope[self.context_name] = stack
+            await self.app(scope, receive, send)

venv/lib/python3.12/site-packages/fastapi/middleware/cors.py

@@ -0,0 +1 @@
+from starlette.middleware.cors import CORSMiddleware as CORSMiddleware  # noqa

venv/lib/python3.12/site-packages/fastapi/middleware/gzip.py

@@ -0,0 +1 @@
+from starlette.middleware.gzip import GZipMiddleware as GZipMiddleware  # noqa

venv/lib/python3.12/site-packages/fastapi/middleware/httpsredirect.py

@@ -0,0 +1,3 @@
+from starlette.middleware.httpsredirect import (  # noqa
+    HTTPSRedirectMiddleware as HTTPSRedirectMiddleware,
+)

venv/lib/python3.12/site-packages/fastapi/middleware/trustedhost.py

@@ -0,0 +1,3 @@
+from starlette.middleware.trustedhost import (  # noqa
+    TrustedHostMiddleware as TrustedHostMiddleware,
+)

venv/lib/python3.12/site-packages/fastapi/middleware/wsgi.py

@@ -0,0 +1,3 @@
+from starlette.middleware.wsgi import (
+    WSGIMiddleware as WSGIMiddleware,
+)  # pragma: no cover # noqa