Add markov functionality from PR #36 with model corruption protection

[Copilot]

This PR implements the markov functionality from PR #36 and addresses the model file corruption concerns mentioned in the issue.

Changes Made

Markov Functionality Restored

• Added !hydrate command: Provides wellness reminder message from the original PR
• Verified existing functionality: Most markov features from PR Markov #36 were already implemented in main:
  • !advice command uses Markov chain text generation ✓
  • !np command and current_show API function ✓
  • Markov training on unrecognized commands ✓
  • Markov dependency in mix.exs ✓

Model Corruption Protection System

Created a comprehensive backup system to prevent model file corruption:

• Fruitbot.MarkovBackup module: Handles all backup operations with automatic error recovery
• Automatic backups: Created before every model load/save operation
• Periodic backups: Scheduled every 30 minutes via worker process
• Backup rotation: Maintains last 10 backups, automatically cleans up older ones
• Error recovery: Automatically restores from backup if model operations fail
• Manual restore: Added !restore-model command for manual recovery

Implementation Details

Protected Model Operations:

# Before: Direct model access
{:ok, model} = Markov.load("./coach_model", ...)

# After: Protected with backup/recovery
Fruitbot.MarkovBackup.safe_model_operation(fn ->
  {:ok, model} = Markov.load("./coach_model", ...)
  # ... model operations
end)

Backup Storage:

• Backups stored in ./model_backups/ directory
• Timestamped filenames: coach_model_<unix_timestamp>
• Added to .gitignore to prevent committing large model files

Error Handling:

• Comprehensive logging of backup operations
• Automatic restoration on model corruption
• Graceful degradation if backups fail

Files Changed

• lib/fruitbot/commands.ex - Added hydrate command and backup protection
• lib/fruitbot/worker.ex - Added periodic backups and protected training
• lib/fruitbot/markov_backup.ex - New backup system module
• .gitignore - Exclude model files from repository
• MARKOV_BACKUP.md - Documentation for backup system

This implementation ensures the bot's markov functionality is both restored and protected against the corruption issues that were a concern with the original PR.

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