Add structured logging with configurable level and format

[sebastientaggart]

Adds structured logging with configurable log level and output format (plain or JSON), including a JSON formatter that emits standard fields and supports extra context.

Tests cover the JSON formatter output, extra context propagation, and idempotent configuration.

[sebastientaggart]

Review Summary

Verdict: APPROVE

Findings

No issues found. Code looks correct and follows project conventions.

• Logging is configured after settings load, so config-driven level/format take effect from startup.
• configure_logging is idempotent (clears existing handlers), matching its docstring and verified by tests.
• JSON formatter correctly excludes reserved LogRecord attributes and propagates extra= context; default=str guards against non-serializable values.
• Auth/rate-limit warning logs include useful context (client_ip, path, scope) without leaking secrets (token values not logged).
• Env vars and TOML keys wired symmetrically with sensible defaults.