feat: add `--watch` flag to `aeorank-cli scan`

[vinpatel]

When iterating on a site, rerunning `aeorank-cli scan` by hand is annoying. A `--watch` flag would rescan on file changes to the local serve target or on a time interval.

Scope

Add `--watch` (and maybe `--watch-interval `, default 5000) to the `scan` command:

• Runs the initial scan as today
• Then re-runs every N ms, or on a filesystem event if the URL is `localhost`
• Prints a diff between runs: "+3 score" / "-1 pass / +1 warn"

Where

CLI commands live in `packages/cli/src/commands/scan.ts`. The command uses Commander; the option surface is already defined there.

Acceptance

• `aeorank-cli scan --watch` works
• Clean Ctrl+C handling (no orphaned processes)
• A test covering the watch loop (can mock the scanner)
• `--help` output shows the new flag with a useful description