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 PR is stacked on #371 and includes that predecessor commit until it lands.

Changes

• Add POST /api/gtd/delegations for 1–100 unique, owned message IDs and an owned contact or null.
• Store contact details and a durable delegation boundary under messages.plugin_annotations.gtd.delegation on every live row in the thread.
• Serialize delegation, transition, and cleanup work per account and thread so concurrent requests cannot create duplicate copies or undo a successful explicit delegation.
• Reconcile non-UIDPLUS copies through one fresh destination sync and 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.
• Keep personless delegation public metadata unchanged while retaining the internal boundary needed to distinguish an older message from a newer reply.

Blocked by #371.

Refs #343 and #368

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