refactor: replace DefaultLimitOffsetPage with LimitOffsetPage in multiple files and update timezone handling to use UTC

backend/app/schemas/boards.py

@@ -34,11 +34,13 @@ class BoardBase(SQLModel):
 class BoardCreate(BoardBase):
     """Payload for creating a board."""
 
-    gateway_id: UUID
+    gateway_id: UUID | None = None
 
     @model_validator(mode="after")
     def validate_goal_fields(self) -> Self:
-        """Require goal details when creating a confirmed goal board."""
+        """Require gateway and goal details when creating a confirmed goal board."""
+        if self.gateway_id is None:
+            raise ValueError(_ERR_GATEWAY_REQUIRED)
         if (
             self.board_type == "goal"
             and self.goal_confirmed

backend/app/schemas/pagination.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TypeVar
+from typing import TYPE_CHECKING, TypeVar
 
 from fastapi import Query
 from fastapi_pagination.customization import CustomizedPage, UseParamsFields
@@ -14,10 +14,15 @@ T = TypeVar("T")
 # Project-wide default pagination response model.
 # - Keep `limit` / `offset` naming (matches existing API conventions).
 # - Cap list endpoints to 200 items per request (matches prior route-level constraints).
-DefaultLimitOffsetPage = CustomizedPage[
-    LimitOffsetPage[T],
-    UseParamsFields(
-        limit=Query(200, ge=1, le=200),
-        offset=Query(0, ge=0),
-    ),
-]
+if TYPE_CHECKING:
+    # Type checkers treat this as a normal generic page type.
+    DefaultLimitOffsetPage = LimitOffsetPage
+else:
+    # Runtime uses project-default query param bounds for all list endpoints.
+    DefaultLimitOffsetPage = CustomizedPage[
+        LimitOffsetPage[T],
+        UseParamsFields(
+            limit=Query(200, ge=1, le=200),
+            offset=Query(0, ge=0),
+        ),
+    ]