Add context-aware requests and configurable timeout

[andreagrandi]

Summary

Closes #11.

Threads context.Context through the Metabase client and CLI so HTTP
requests can be cancelled and time-bounded consistently, as requested in
issue #11.

• Client request methods accept a context and build requests with
  http.NewRequestWithContext; Do reports clear request timed out /
  request canceled errors while still wrapping context.DeadlineExceeded
  and context.Canceled for programmatic detection.
• The root context is cancelled on SIGINT/SIGTERM via signal.NotifyContext,
  so commands abort cleanly on Ctrl+C.
• New --timeout flag (duration, default 30s, 0 disables) bounds each
  command's API requests, replacing the fixed http.Client timeout.
• Timeout and cancellation errors are classified as TIMEOUT_ERROR /
  CANCELED_ERROR for --error-format json.

Existing command output and PII redaction behavior are unchanged.

Changelog

• Added an entry under ## [Unreleased] in CHANGELOG.md
• No changelog entry needed

Test plan

• make fmt, make vet, make test, make build all pass.
• New tests/request_context_test.go covers Get/Post cancellation, Get
  timeout, context propagation through a high-level client method, and an
  end-to-end --timeout CLI test; error-classification tests cover the new
  error types.
• Smoke-tested against a real Metabase instance: database/table/field/card/
  dashboard/query/search commands return expected output.
• --timeout 1ms produces a structured TIMEOUT_ERROR.
• Verified Ctrl+C aborts an in-flight SELECT pg_sleep(10) with
  request canceled: ... context canceled and a non-zero exit.