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/images/validation.py

@@ -3,21 +3,11 @@
 import magic
 from fastapi import HTTPException, UploadFile, status
 
-# Maximum file size: 50MB
-MAX_FILE_SIZE = 52_428_800
-
-# Allowed MIME types
-ALLOWED_MIME_TYPES = {
-    "image/jpeg",
-    "image/jpg",
-    "image/png",
-    "image/gif",
-    "image/webp",
-    "image/svg+xml",
-}
-
-# Allowed file extensions
-ALLOWED_EXTENSIONS = {".jpg", ".jpeg", ".png", ".gif", ".webp", ".svg"}
+from app.core.constants import (
+    ALLOWED_EXTENSIONS,
+    ALLOWED_MIME_TYPES,
+    MAX_IMAGE_SIZE,
+)
 
 
 async def validate_image_file(file: UploadFile) -> bytes:
@@ -50,10 +40,10 @@ async def validate_image_file(file: UploadFile) -> bytes:
     if file_size == 0:
         raise HTTPException(status_code=status.HTTP_400_BAD_REQUEST, detail="Empty file uploaded")
 
-    if file_size > MAX_FILE_SIZE:
+    if file_size > MAX_IMAGE_SIZE:
         raise HTTPException(
             status_code=status.HTTP_413_REQUEST_ENTITY_TOO_LARGE,
-            detail=f"File too large. Maximum size is {MAX_FILE_SIZE / 1_048_576:.1f}MB",
+            detail=f"File too large. Maximum size is {MAX_IMAGE_SIZE / 1_048_576:.1f}MB",
         )
 
     # Validate file extension