feat: show GTD state indicators in inbox rows

[salmonumbrella]

Summary

Ports the inbox GTD state indicators from closed PR #330 onto the plugin boundary described in #368. Ordinary inbox and search rows can now show every GTD state on the thread, with the oldest relevant classification date, without putting GTD-specific joins or fields into MailFlow's core message query.

This is a stacked draft. It is blocked by #370 and currently includes that PR's commit; once #370 lands, this diff collapses to the indicator layer.

Changes

• Add a bounded, plugin-owned metadata endpoint for up to 100 visible messages at a time.
• Expose only a neutral message-row-meta slot from the core message list. GTD owns the labels, colors, ordering, dates, and account gating.
• Resolve state metadata across every live copy of the same message and thread, including custom GTD folder mappings.
• Group and chunk visible-row requests per account, ignore superseded responses, and keep optimistic classification and undo state coherent with server refreshes.
• Refresh indicators after GTD WebSocket events, reconnects, folder-map changes, classification, undo, and unclassification.
• Show compact state chips and a conservative age for multi-state threads.

Testing

• Backend: npm test — 1,127 tests passed across 63 files.
• Backend: npm run lint and npm run lint:plugins.
• Frontend: npm test — 1,592 tests passed.
• Frontend: npm run lint and npm run build.
• Regression coverage includes account ownership, request bounds, custom folder mappings, cross-copy/thread metadata, canonical state order, request races, optimistic classify/undo, stale cache invalidation, and indicator rendering.
• RoboRev reviewed the final commit. Its cache-coherency findings were reproduced and fixed; the remaining reported unified-inbox finding was checked against the actual condition and does not apply.

Blocked by #370.

Refs #368

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