chore(feat): impl. delete challenge endpoint

[nully0x]

This PR implements DELETE endpoint that allows administrators to remove challenges from the platform.

Changes

• Added DELETE endpoint /api/challenge?id={uuid}
• Added challenge deletion functionality to Challenge model
• Added new error types for delete operations
• Implemented admin-only authorization check

API Usage

DELETE /api/challenge/delete?id=550e8400-e29b-41d4-a716-446655440000
x-session-token: hxckr_your_session_token

Response

Success (200):

{
    "status": "success",
    "message": "Challenge deleted successfully"
}

Checks

• Requires valid session token
• Limited to admin users only
• Validates challenge existence before deletion

Testing

• [ x] Verify admin-only access
• [x ] Verify successful challenge deletion
• [ x] Verify error handling for non-existent challenges
• [x ] Verify unauthorized access handling

[Extheoisah]

No need to return anything in the Ok response since it's not being used anywhere

Suggested change

	Ok(count) => Ok(count),
	Ok(_) => (),

[Extheoisah]

No need for a return type if we're not returning anything based on my recommendation.

Suggested change

	pub fn delete(connection: &mut PgConnection, challenge_id: &Uuid) -> Result<usize> {
	pub fn delete(connection: &mut PgConnection, challenge_id: &Uuid) -> Result<()> {