Structured JSON logging

[pgray]

Summary

Switch from the current plain-text tracing output to structured JSON logging. Makes logs parseable by log aggregators (Loki, CloudWatch, etc.) and easier to filter/search.

Current state

Uses tracing_subscriber::fmt() with default text format and env-filter.

Proposed

• Add tracing-subscriber JSON format layer (already has the feature: json)
• Toggle via env var: MAYL_LOG_FORMAT=json (default: text for local dev)
• Keep RUST_LOG / env-filter for level control
• Structured fields already in place (info!(user = %smtp_user, ...)) — just need the formatter swap

Implementation

let subscriber = tracing_subscriber::fmt()
    .with_env_filter(
        EnvFilter::try_from_default_env()
            .unwrap_or_else(|_| "mayl=info".parse().unwrap()),
    );

if env_or("MAYL_LOG_FORMAT", "text") == "json" {
    subscriber.json().init();
} else {
    subscriber.init();
}

Add "json" to tracing-subscriber features in Cargo.toml.

Verification

• MAYL_LOG_FORMAT=json cargo run outputs JSON lines
• cargo run still outputs plain text (default)
• All existing info!/warn!/error! fields appear as JSON keys