feat: graceful shutdown on SIGINT/SIGTERM

[prochac]

Propagate context through the fetch layer to cancel in-flight requests on shutdown, and drain the HTTP server before exit.

Pull Request Template

Description

Adds graceful shutdown on SIGINT/SIGTERM. The exporter derives a context from
signal.NotifyContext; on signal it shuts down the HTTP server with a bounded
drain and cancels in-flight Cloudflare requests instead of being killed mid-flight.

Also:

• run the scrape loop synchronously, so ticks coalesce instead of piling up
  overlapping scrapes (also fixes a per-iteration time.NewTicker leak in the
  old loop)
• suppress expected context.Canceled errors in fetch logs so shutdown is quiet

Type of Change

• New feature (non-breaking change which adds functionality)

Testing

• I have run make pr-tests and all tests pass
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

fmt-check, lint (golangci-lint: 0 issues), and build pass. The test
step (run_e2e.sh) couldn't run in my environment — it requires the venom
binary and a .env file, neither available here. Verified manually instead:
sent SIGTERM and confirmed clean drain (Shutdown complete, exit 0); with an
in-flight request (routed through a hanging proxy) confirmed the
context.Canceled error is suppressed (0 error log lines).

Code Quality

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation

Additional Notes

No documentation/config changes needed — cf_timeout and existing flags are
unchanged. Shutdown drain is bounded at 10s; in-flight scrapes are best-effort
and abandoned on exit (their results are discarded anyway).