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/uvloop/includes/system.pxd

@@ -0,0 +1,96 @@
+from libc.stdint cimport int8_t, uint64_t
+
+cdef extern from "arpa/inet.h" nogil:
+
+    int ntohl(int)
+    int htonl(int)
+    int ntohs(int)
+
+
+cdef extern from "sys/socket.h" nogil:
+
+    struct sockaddr:
+        unsigned short sa_family
+        char           sa_data[14]
+
+    struct addrinfo:
+        int            ai_flags
+        int            ai_family
+        int            ai_socktype
+        int            ai_protocol
+        size_t         ai_addrlen
+        sockaddr*      ai_addr
+        char*          ai_canonname
+        addrinfo*      ai_next
+
+    struct sockaddr_in:
+        unsigned short sin_family
+        unsigned short sin_port
+        # ...
+
+    struct sockaddr_in6:
+        unsigned short sin6_family
+        unsigned short sin6_port
+        unsigned long  sin6_flowinfo
+        # ...
+        unsigned long  sin6_scope_id
+
+    struct sockaddr_storage:
+        unsigned short ss_family
+        # ...
+
+    const char *gai_strerror(int errcode)
+
+    int socketpair(int domain, int type, int protocol, int socket_vector[2])
+
+    int setsockopt(int socket, int level, int option_name,
+                   const void *option_value, int option_len)
+
+
+cdef extern from "sys/un.h" nogil:
+
+    struct sockaddr_un:
+        unsigned short sun_family
+        char*          sun_path
+        # ...
+
+
+cdef extern from "unistd.h" nogil:
+
+    ssize_t write(int fd, const void *buf, size_t count)
+    void _exit(int status)
+
+
+cdef extern from "pthread.h":
+
+    int pthread_atfork(
+        void (*prepare)(),
+        void (*parent)(),
+        void (*child)())
+
+
+cdef extern from "includes/compat.h" nogil:
+
+    cdef int EWOULDBLOCK
+
+    cdef int PLATFORM_IS_APPLE
+    cdef int PLATFORM_IS_LINUX
+
+    struct epoll_event:
+        # We don't use the fields
+        pass
+
+    int EPOLL_CTL_DEL
+    int epoll_ctl(int epfd, int op, int fd, epoll_event *event)
+    object MakeUnixSockPyAddr(sockaddr_un *addr)
+
+
+cdef extern from "includes/fork_handler.h":
+
+    uint64_t MAIN_THREAD_ID
+    int8_t MAIN_THREAD_ID_SET
+    ctypedef void (*OnForkHandler)()
+    void handleAtFork()
+    void setForkHandler(OnForkHandler handler)
+    void resetForkHandler()
+    void setMainThreadID(uint64_t id)