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/websockets/sync/utils.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+import time
+
+
+__all__ = ["Deadline"]
+
+
+class Deadline:
+    """
+    Manage timeouts across multiple steps.
+
+    Args:
+        timeout: Time available in seconds or :obj:`None` if there is no limit.
+
+    """
+
+    def __init__(self, timeout: float | None) -> None:
+        self.deadline: float | None
+        if timeout is None:
+            self.deadline = None
+        else:
+            self.deadline = time.monotonic() + timeout
+
+    def timeout(self, *, raise_if_elapsed: bool = True) -> float | None:
+        """
+        Calculate a timeout from a deadline.
+
+        Args:
+            raise_if_elapsed: Whether to raise :exc:`TimeoutError`
+                if the deadline lapsed.
+
+        Raises:
+            TimeoutError: If the deadline lapsed.
+
+        Returns:
+            Time left in seconds or :obj:`None` if there is no limit.
+
+        """
+        if self.deadline is None:
+            return None
+        timeout = self.deadline - time.monotonic()
+        if raise_if_elapsed and timeout <= 0:
+            raise TimeoutError("timed out")
+        return timeout