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/constants.py

@@ -0,0 +1,38 @@
+"""Application-wide constants."""
+
+# File upload limits
+MAX_IMAGE_SIZE = 52_428_800  # 50MB in bytes
+MAX_ZIP_SIZE = 209_715_200  # 200MB in bytes
+
+# Image processing
+MAX_IMAGE_DIMENSION = 10_000  # Max width or height in pixels
+THUMBNAIL_SIZES = {
+    "low": 800,  # For slow connections (<1 Mbps)
+    "medium": 1600,  # For medium connections (1-5 Mbps)
+    "high": 3200,  # For fast connections (>5 Mbps)
+}
+
+# Pagination defaults
+DEFAULT_PAGE_SIZE = 50
+MAX_PAGE_SIZE = 100
+
+# Board limits
+MAX_BOARD_TITLE_LENGTH = 255
+MAX_BOARD_DESCRIPTION_LENGTH = 1000
+MAX_IMAGES_PER_BOARD = 1000
+
+# Authentication
+TOKEN_EXPIRE_HOURS = 168  # 7 days
+PASSWORD_MIN_LENGTH = 8
+
+# Supported image formats
+ALLOWED_MIME_TYPES = {
+    "image/jpeg",
+    "image/jpg",
+    "image/png",
+    "image/gif",
+    "image/webp",
+    "image/svg+xml",
+}
+
+ALLOWED_EXTENSIONS = {".jpg", ".jpeg", ".png", ".gif", ".webp", ".svg"}