Fix Slack auth filter not halting on invalid signature

[JuanVqz]

Problem

valid_slack_request? in Slack::ApplicationController returned a boolean but never called render or head, so the before_action never halted the filter chain. Every Slack endpoint accepted any request regardless of signature validity.

# Before — always proceeds to action
def valid_slack_request?
  @verified ||= verify_slack_signature  # just a boolean, no halt
end

Fix

Added head :unauthorized and return to halt the chain when verification fails, consistent with the pattern used in #66:

def valid_slack_request?
  head :unauthorized and return unless verify_slack_signature
end

Tests

Added Slack::PuzzlesControllerTest covering:

• Requests with an invalid signature → 401
• Requests with an expired timestamp → 401
• Requests with a valid signature → allowed through (200)

[JuanVqz]

Superseded by #71 which addresses this and three other related bugs with a more robust design.