Add global cleanup policy configuration

[mxpv]

Summary

• Implements global cleanup policy configuration as requested in Disable cleanup for all feeds? #214
• Changes per-feed cleanup policy to optional pointer to enable inheritance
• Adds comprehensive fallback logic and testing

Changes Made

Configuration Structure

• Add Cleanup *feed.Cleanup field to main Config struct
• Change per-feed Clean Cleanup to Clean *Cleanup (pointer for optional inheritance)
• Add fallback logic in applyDefaults() to apply global policy when feed doesn't specify its own

Implementation Details

• Add nil check in cleanup function to handle feeds without cleanup policies
• Maintain backward compatibility - existing configurations continue to work unchanged
• Global policy is applied only to feeds that don't specify their own cleanup policy

Documentation & Testing

• Update config.toml.example with global cleanup configuration example
• Add comprehensive test suite covering all inheritance scenarios:
  • Global cleanup policy inheritance by feeds without their own policy
  • No global cleanup policy scenario
  • Per-feed cleanup overriding global cleanup
• Update existing test to handle pointer change

Usage Example

# Global cleanup policy (applies to all feeds without explicit cleanup)
[cleanup]
keep_last = 50

[feeds]
  # This feed inherits the global policy (keep_last = 50)
  [feeds.FEED1]
  url = "https://youtube.com/channel/example1"
  
  # This feed overrides the global policy
  [feeds.FEED2]
  url = "https://youtube.com/channel/example2"
  clean = { keep_last = 10 }

Test Results

✅ All existing tests pass
✅ New comprehensive test suite added
✅ Linting passes
✅ Build successful

Fixes #214