feat: add core application constants, ownership verification, and repository utilities

- Introduced application-wide constants for file uploads, image processing, pagination, and authentication in `constants.py`.
- Implemented synchronous and asynchronous board ownership verification functions in `ownership.py`.
- Created a base repository class with common CRUD operations in `repository.py`.
- Added standard response utilities for error and success messages in `responses.py`.
- Refactored image validation to utilize constants for file size and MIME types.
- Enhanced frontend components with consistent styling and validation utilities for forms.
- Established global styles for buttons, forms, loading indicators, and messages to ensure a cohesive UI experience.

backend/app/core/ownership.py

@@ -0,0 +1,69 @@
+"""Ownership verification utilities."""
+
+from uuid import UUID
+
+from fastapi import HTTPException, status
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import Session
+
+from app.database.models.board import Board
+
+
+def verify_board_ownership_sync(db: Session, board_id: UUID, user_id: UUID) -> Board:
+    """
+    Verify board ownership (synchronous).
+
+    Args:
+        db: Database session
+        board_id: Board UUID
+        user_id: User UUID
+
+    Returns:
+        Board instance if owned by user
+
+    Raises:
+        HTTPException: 404 if board not found or not owned by user
+    """
+    stmt = select(Board).where(
+        Board.id == board_id,
+        Board.user_id == user_id,
+        Board.is_deleted == False,  # noqa: E712
+    )
+
+    board = db.execute(stmt).scalar_one_or_none()
+
+    if not board:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=f"Board {board_id} not found")
+
+    return board
+
+
+async def verify_board_ownership_async(db: AsyncSession, board_id: UUID, user_id: UUID) -> Board:
+    """
+    Verify board ownership (asynchronous).
+
+    Args:
+        db: Async database session
+        board_id: Board UUID
+        user_id: User UUID
+
+    Returns:
+        Board instance if owned by user
+
+    Raises:
+        HTTPException: 404 if board not found or not owned by user
+    """
+    stmt = select(Board).where(
+        Board.id == board_id,
+        Board.user_id == user_id,
+        Board.is_deleted == False,  # noqa: E712
+    )
+
+    result = await db.execute(stmt)
+    board = result.scalar_one_or_none()
+
+    if not board:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=f"Board {board_id} not found")
+
+    return board