Add schemas for presigned URL requests and responses

Introduced two Pydantic models: `PresignedUrlRequest` and `PresignedUrlResponse`. These define the request and response structures for presigned URL generation, including fields for filenames, content types, and expiration details. This provides a clear contract for handling presigned URL functionality.

backend/app/schemas/presigned_urls.py

@@ -0,0 +1,24 @@
+from pydantic import BaseModel, Field
+
+
+class PresignedUrlRequest(BaseModel):
+    """Request model for generating presigned URLs."""
+    filename: str = Field(..., description="Original filename of the image")
+    content_type: str = Field(..., description="Content type of the file (e.g., image/jpeg)")
+    folder: str = Field("images", description="Folder to store the file in")
+
+    class Config:
+        schema_extra = {
+            "example": {
+                "filename": "my-image.jpg",
+                "content_type": "image/jpeg",
+                "folder": "event-themes"
+            }
+        }
+
+
+class PresignedUrlResponse(BaseModel):
+    """Response model for presigned URL generation."""
+    upload_url: str = Field(..., description="URL to upload the file to")
+    file_url: str = Field(..., description="URL where the file will be accessible after upload")
+    expires_in: int = Field(..., description="Time in seconds until the upload URL expires")