feat: replace println with tracing framework

[echobt]

This PR replaces the usage of println! for diagnostic output with the tracing framework. It introduces structured logging capabilities, allowing users to control log levels and output destinations.

Changes

• Added tracing, tracing-subscriber, and tracing-appender dependencies.
• Implemented logging initialization in src/logging.rs.
• Replaced println! calls with info! and warn! macros in src/core/indexer.rs, src/watcher.rs, and src/server/api.rs.
• Added --log-level and --log-file CLI arguments to control logging behavior.
• Preserved UI-related println! calls (e.g., prompts, progress bars, search results) to maintain user interaction.

Benefits

• Configurable Log Levels: Users can specify the verbosity of logs (error, warn, info, debug, trace).
• File Logging: Logs can be directed to a file for persistent storage and analysis.
• Structured Logging: Provides a foundation for machine-readable logs and integration with observability tools.
• Production Debugging: Facilitates easier debugging in production environments by enabling detailed logging without cluttering stdout.