Observability: tracing integration, log viewer, event schema

[thedavidweng]

Problem

~30 eprintln!/println! calls scattered across the Rust codebase with no structured logging. No in-app log viewer. The NDJSON event schema exists in docs but isn't consistently emitted.

Scope

• Introduce tracing + tracing-subscriber with JSONL output
• Replace ad-hoc eprintln!/println! calls with structured tracing macros
• Log file rotation with BACKEND_LOG_RETAIN_COUNT
• Formalize NDJSON event schema with JSON Schema validation
• Unified event emitter for CLI and GUI
• In-app log viewer in Settings
• Log line-level filtering by level and event type

Acceptance criteria

• No bare eprintln!/println! in src-tauri/src/ (except tests)
• Logs rotate automatically, keeping the configured number of files
• Event schema doc has a machine-readable JSON Schema
• Settings has a log viewer with level filtering