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/responses.py

@@ -0,0 +1,75 @@
+"""Standard response utilities."""
+
+from typing import Any
+
+from fastapi import status
+
+
+class ErrorResponse:
+    """Standard error response formats."""
+
+    @staticmethod
+    def not_found(resource: str = "Resource") -> dict[str, Any]:
+        """404 Not Found response."""
+        return {
+            "status_code": status.HTTP_404_NOT_FOUND,
+            "detail": f"{resource} not found",
+        }
+
+    @staticmethod
+    def forbidden(message: str = "Access denied") -> dict[str, Any]:
+        """403 Forbidden response."""
+        return {
+            "status_code": status.HTTP_403_FORBIDDEN,
+            "detail": message,
+        }
+
+    @staticmethod
+    def unauthorized(message: str = "Authentication required") -> dict[str, Any]:
+        """401 Unauthorized response."""
+        return {
+            "status_code": status.HTTP_401_UNAUTHORIZED,
+            "detail": message,
+            "headers": {"WWW-Authenticate": "Bearer"},
+        }
+
+    @staticmethod
+    def bad_request(message: str) -> dict[str, Any]:
+        """400 Bad Request response."""
+        return {
+            "status_code": status.HTTP_400_BAD_REQUEST,
+            "detail": message,
+        }
+
+    @staticmethod
+    def conflict(message: str) -> dict[str, Any]:
+        """409 Conflict response."""
+        return {
+            "status_code": status.HTTP_409_CONFLICT,
+            "detail": message,
+        }
+
+
+class SuccessResponse:
+    """Standard success response formats."""
+
+    @staticmethod
+    def created(data: dict[str, Any], message: str = "Created successfully") -> dict[str, Any]:
+        """201 Created response."""
+        return {
+            "message": message,
+            "data": data,
+        }
+
+    @staticmethod
+    def ok(data: dict[str, Any] | None = None, message: str = "Success") -> dict[str, Any]:
+        """200 OK response."""
+        response = {"message": message}
+        if data:
+            response["data"] = data
+        return response
+
+    @staticmethod
+    def no_content() -> None:
+        """204 No Content response."""
+        return None