LinkedIn: per-destination posting targets in the composer

[Adron]

Scope

Let the user choose which LinkedIn destinations a post goes to, instead of a single on/off toggle.

API

• GET /api/linkedin/targets — available destinations (live-verified 200 {"targets":[]}).
• GET/PUT /api/linkedin/posting-targets — the user's saved default selection.
• POST /api/messages takes linkedInTargets: [{kind: "personal"} | {kind: "orgPage", pageId}].

Prefer linkedInTargets (plural). The spec marks linkedInTarget as "superseded singular form, still accepted for older clients" — do not build on it.

Acceptance criteria

• Service methods for targets and posting-targets.
• Composer advanced panel lists personal + org-page destinations with individual toggles.
• Selection is sent as linkedInTargets; the legacy singular is never sent.
• Saved defaults from GET /api/linkedin/posting-targets pre-select, and PUT persists a change.
• Empty targets degrade to the personal destination without an error.
• Subscriber-gated with the upgrade-needed state.