Implement upvote functionality for "+" messages

[konard]

Summary

Implements the ability to treat a simple "+" message as an upvote for the previous non-command message in the VK bot, addressing issue #7.

Changes Made

• New Pattern: Added UPVOTE_PREVIOUS regex pattern to match standalone "+" messages (with optional whitespace)
• Message History: Added get_conversation_messages() method to retrieve recent conversation history via VK API
• Upvote Logic: Implemented upvote_previous() method that:
  • Finds the most recent non-command message in the conversation
  • Applies karma upvote using existing collective vote mechanism (+0 karma)
  • Validates user permissions and voting limits
  • Prevents self-upvoting
  • Shows appropriate error messages when needed
• Integration: Registered the new pattern and handler in the bot's command system
• Testing: Added comprehensive pattern validation tests

Technical Details

• Uses existing karma system infrastructure (collective votes, time limits, user validation)
• Leverages VK API messages.getHistory to analyze conversation history
• Searches backwards through up to 20 recent messages to find the target
• Filters out bot messages, system messages, and all registered commands
• Maintains all existing karma system constraints and protections

Test Results

• Pattern matching: ✅ All test cases pass
• Python syntax validation: ✅ No errors
• Integration with existing commands: ✅ Successfully registered

🤖 Generated with Claude Code

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Resolves #7