feat(gtd): 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.

The prerequisite plugin-boundary work from #370 is merged, and this branch is rebased onto current main.

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• Group and chunk visible-row requests per account, ignore superseded responses, and keep optimistic classification and undo state coherent with server refreshes.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.
• Treat partial metadata responses as retryable and keep retries bounded and cancelable when no consumers remain.

Refs #368

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[salmonumbrella]

@maathimself 3 GTD PRs ready for you.