Social account linking (OAuth) and full cross-post destination support #38

Where this stands

Cross-posting to already-linked networks works: the composer reads GET /api/user/identities
and sends mastodonProviderIds[] + crossPostTo{Bluesky,LinkedIn,Twitter}.

What is missing is everything around it — linking an account in the first place, unlinking,
re-verifying, and LinkedIn's destination model.

Blocked

The OAuth connect flow needs registered redirect URIs for the Android app before it can be
built. Everything else in this epic can proceed independently.

Unused API surface

• GET /api/auth/{bluesky,github,mastodon,twitter,linkedin}/authorize|callback|status
• POST /api/auth/{provider}/link
• POST /api/user/identities/verify
• GET|PUT /api/linkedin/posting-targets, GET /api/linkedin/targets, POST /api/linkedin/sync-pages
• GET /api/organizations/{id}/linkedin/status, PUT …/assignments, DELETE …/credential, POST …/sync-pages
• linkedInTargets[] and linkedInLinkAsFirstComment on POST /api/messages

Reference: /help/cross-posting.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.