feat(gtd): add delegated contact API

[salmonumbrella]

Summary

Ports the backend half of delegated-contact GTD state from closed PR #343 onto the plugin boundary described in #368. A caller can now label one or more owned threads as Delegated and attach a durable contact snapshot without adding a GTD table, migration, or command-engine dependency.

This is a stacked draft. It is blocked by #371, which is blocked by #370. It currently includes both earlier commits; the diff collapses to this API layer as those PRs land.

Changes

• Add POST /api/gtd/delegations for 1–100 unique, owned message IDs and an owned contact or null.
• Store contactId, display name, primary email, and timestamps under messages.plugin_annotations.gtd.delegation on every live row in the thread.
• Serialize work per account and thread so concurrent requests create at most one Delegated copy.
• Reconcile non-UIDPLUS copies by syncing the destination once and taking a strict UID set difference. Ambiguous outcomes are reported without guessing.
• Compensate annotation failures only when this request can identify the exact label copy it created. Pre-existing labels are never removed.
• Inherit delegation snapshots onto newly synced rows and clear them only after the final Delegated copy disappears.
• Include the contact snapshot in the plugin-owned row metadata response.

Testing

• Backend: npm test — 1,157 tests passed across 66 files.
• Backend: npm run lint and npm run lint:plugins.
• Coverage includes ownership boundaries, duplicate IDs, thread grouping, contact clearing, exact compensation, pre-existing labels, concurrent calls, UIDPLUS and non-UIDPLUS reconciliation, ambiguous outcomes, route validation, annotation inheritance, and final-label cleanup.
• RoboRev reviewed the final API commit with no issues found.

Blocked by #371.

Refs #343 and #368

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