a11y: make database action links keyboard-operable

[xpoes123]

The download (TXT / CSV / JSON) and section-jump links in client/db/index.jsx were <a> elements without an href, so they were invisible to keyboard navigation. This PR makes all five interactive without changing their appearance or click behavior.

Problem

In client/db/index.jsx, five <a class="... clickable"> elements relied solely on onClick handlers with no href. Anchor elements without href are excluded from the tab order, so keyboard users could not download results or jump between the tossup and bonus sections.

Changes

• Adds role="button", tabIndex={0}, and an onKeyDown handler (activating on Enter and Space) to each of the five affected links: the TXT, CSV, and JSON download links and the "Jump to bonuses" / "Jump to tossups" scroll links.
• Existing onClick handlers and the clickable CSS class are preserved as-is; visual appearance is unchanged.

Risk & testing

The change is purely additive — no existing logic is altered. The onKeyDown guard checks event.key === 'Enter' || event.key === ' ' and calls event.preventDefault() before the action to avoid page-scroll on Space. semistandard and node --check pass.