Organizations: LinkedIn organization integration

[Adron]

Endpoints

• GET /api/organizations/{id}/linkedin/status
• POST /api/organizations/{id}/linkedin/sync-pages
• PUT /api/organizations/{id}/linkedin/assignments
• DELETE /api/organizations/{id}/linkedin/credential
• plus GET /api/auth/linkedin/org-authorize and /org-callback for the OAuth leg

This is how an organization's LinkedIn pages become available as posting targets — the org-level counterpart to the per-user LinkedIn targeting in the Cross-posting epic.

Acceptance criteria

• All four org LinkedIn endpoints implemented; shapes verified live (the test account has no org LinkedIn credential — expect empty state).
• An org detail shows its LinkedIn link state, with connect (browser handoff) and disconnect.
• Sync-pages refreshes available pages; assignments maps pages to the org.
• Owner/admin gated, consistent with the rest of the Organizations view.
• OAuth opens the OS browser — no WebView2.

[Adron]

Useful find from #108/PR #154: GET /api/linkedin/posting-targets returns {"targets":[],"orgScopeMissing":false} — that orgScopeMissing flag is undocumented in the spec and is presumably how the server tells you the LinkedIn OAuth grant lacks the organization scope. That's likely the signal this issue needs to distinguish "no org pages" from "we can't see your org pages".

Also relevant: on an account with no LinkedIn identity, GET /api/linkedin/targets and /posting-targets both return 200 with an empty array, while POST /api/linkedin/sync-pages returns 400 code:"not_linked". So don't infer link state from an empty collection — read /api/user/identities. SyncLinkedInPagesAsync and GetLinkedInTargetsAsync already exist on that branch.