feat(gtd): show GTD state indicators in inbox rows

[salmonumbrella]

Summary

@maathimself, this brings #371 back onto current main now that #493, #535 and #538 have landed. Inbox and search rows show each GTD state on the message or thread. The backend metadata endpoint and frontend chips stay behind the plugin boundary from #368.

Waiting chips show the oldest labeled message’s age, because MailFlow does not store when a label was applied. They turn stale at 15 whole days.

Changes

• Add a bounded, account-scoped label reader and plugin-owned metadata endpoint, including custom folder mappings and live copies matched by thread or Message-ID.
• Add a neutral message-row slot for flat and threaded rows; GTD owns visibility, state order, colors and accessible age labels.
• Batch visible rows per account, publish healthy batches independently, and guard stale responses, retries and session teardown.
• Refresh after classification, undo, Done, unclassification, GTD events, reconnects and folder-map changes.

Replaces #371. Refs #368. #372 and #373 remain separate follow-ups in the maintainer’s requested order.

Testing

• Node 22 CI-equivalent gate: backend lint, plugin-boundary lint, syntax and 2,421 tests; frontend lint, production build and 2,840 tests.
• All 32 optional PostgreSQL tests passed against a disposable database, including metadata, thread counts and reply-draft lookup/ingestion.
• Regression checks failed against the main integration and reintroduced cache/SQL defects, then passed with the feature restored.
• Both production dependency audits passed the required high-severity threshold. Existing moderate advisories remain in qs and React Router; dependencies and lockfiles are unchanged.

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