fix: transfers and standardize file checks

[nullable-eth]

Pull Request Description

🐛 Bug Fixes & Improvements

Fixed JSON Unmarshaling Error for TV Shows

• Issue: Plex API sometimes returns subtitleMode as string instead of integer, causing parsing failures
• Solution: Created FlexibleInt type that handles both string and integer values
• Impact: Resolves sync failures for TV shows with user set values at the tv show level (change subtitle display for specific tv show)

Fixed Redundant Transfer Method Logging

• Issue: Duplicate "High-performance file transfer enabled" messages in logs
• Solution: Removed redundant log from orchestrator, kept transfer-specific logging in transfer module
• Impact: Cleaner, more focused log output

Simplified Environment Variable Names

• Issue: SSH variables had inconsistent OPT_ prefixes
• Solution: Standardized to SSH_USER, SSH_PASSWORD, SSH_PORT, SSH_KEY_PATH
• Impact: More intuitive configuration, updated documentation and examples

Enhanced Path Mapping for Same-Volume Mounting

• Issue: Path mapping failed when mounting same volumes as Plex server
• Solution: Improved logic to handle scenarios where SOURCE_REPLACE_TO is empty
• Implementation:

  # New: Same-volume mounting support
  SOURCE_REPLACE_FROM: "/data/Media"  # Strip this prefix
  SOURCE_REPLACE_TO: ""               # Empty = use Plex paths as-is  
  DEST_ROOT_DIR: "/mnt/storage"       # Destination root

• Result: /data/Media/Movies/Action/movie.mkv → /mnt/storage/Movies/Action/movie.mkv

Fixed File Transfer Issues with Special Characters

• Issue: SCP/rsync failed on files with apostrophes and special characters
• Root Causes:
  1. SCP wasn't using sshpass for password authentication
  2. Incorrect shell escaping in exec.Command usage
• Solutions:
  1. Properly implemented sshpass wrapper for password auth
  2. Removed shell quotes (Go's exec.Command handles argument separation automatically)
• Impact: Files like Miller's Girl (2024).mp4 and The Internet's Own Boy.mkv now transfer successfully

📚 Documentation Updates

• Updated README with new environment variable names
• Added sshpass requirement for password authentication
• Documented same-volume mounting configuration examples
• Clarified path mapping behavior and use cases

🔧 Technical Details

New Types:

type FlexibleInt struct {
    Value int
}
// Handles both string and integer JSON values

Enhanced Path Mapping Logic:

• Supports traditional path replacement: SOURCE_REPLACE_FROM + SOURCE_REPLACE_TO
• Supports same-volume mounting: SOURCE_REPLACE_FROM + empty SOURCE_REPLACE_TO
• Backward compatible with existing configurations

Transfer Method Improvements:

• Proper sshpass integration for both SCP and rsync
• Correct argument handling for exec.Command
• Enhanced error logging and debugging

✅ Testing

• All builds pass successfully
• Path mapping tested with complex scenarios
• Environment variable changes validated
• Transfer methods verified for special character handling

🚀 Benefits

• Reliability: Eliminates sync failures from JSON parsing and special characters
• Flexibility: Supports multiple volume mounting strategies
• Usability: Simplified configuration with cleaner environment variables
• Maintainability: Reduced redundant logging and improved error handling

---

Breaking Changes:

• Environment variables: OPT_SSH_* → SSH_* (documented migration path provided)

Compatibility:

• All existing path mapping configurations continue to work
• New same-volume mounting is opt-in via configuration