observability: replace log.Printf with structured slog logging

[more-shubham]

All logging currently uses log.Printf which outputs unstructured plain text. This cannot be parsed by any observability stack (ELK, Loki, Datadog).

Scope:

• Replace all log.Printf calls across worker.go, engine.go, collector.go with slog
• Add structured fields per log call: worker_id, file, duration_ms, bytes_saved, error
• Add -log-format flag to main.go accepting text (default) or json
• JSON mode should produce one parseable object per line

Example output in JSON mode:

{"time":"2026-02-18T10:30:00Z","level":"INFO","msg":"image processed","worker_id":1,"file":"photo.jpg","duration_ms":492,"bytes_saved":8432}

Acceptance Criteria:

• Zero log.Printf remaining in codebase
• -log-format=json produces valid JSON per line
• Existing -verbose flag behaviour unchanged