feat(user-preference): add user preference support (#55)

APIs:

- GET `/api/private/user/preferences`: Get current user's preferences.
- PATCH `/api/private/user/preferences`: Modify current user's preferences. (body: Preferences)
- PUT `/api/private/user/preferences`: Overwrite current user's preferences. (body: Preferences)
- DELETE `/api/private/user/preferences`: Reset current user's preferences. (body: list[str])
  - body specifies the content to be reset. If body is empty, reset all preferences.

User:

- `User.g0v0_playmode`: show the special ruleset like `OSURX`, and custom rulesets in the future.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

app/utils.py

@@ -272,3 +272,30 @@ bg_tasks = BackgroundTasks()
 
 def utcnow() -> datetime:
     return datetime.now(tz=UTC)
+
+
+def hex_to_hue(hex_color: str) -> int:
+    """Convert a hex color string to a hue value (0-360)."""
+    hex_color = hex_color.lstrip("#")
+    if len(hex_color) != 6:
+        raise ValueError("Invalid hex color format. Expected format: RRGGBB")
+
+    r = int(hex_color[0:2], 16) / 255.0
+    g = int(hex_color[2:4], 16) / 255.0
+    b = int(hex_color[4:6], 16) / 255.0
+
+    max_c = max(r, g, b)
+    min_c = min(r, g, b)
+    delta = max_c - min_c
+
+    if delta == 0:
+        return 0  # Achromatic (grey)
+
+    if max_c == r:
+        hue = (60 * ((g - b) / delta) + 360) % 360
+    elif max_c == g:
+        hue = (60 * ((b - r) / delta) + 120) % 360
+    else:  # max_c == b
+        hue = (60 * ((r - g) / delta) + 240) % 360
+
+    return int(hue)