Harden Bluesky publishing for production use

Postline can already publish to Bluesky (connect via app password, publish, link previews, timeline), but the flow needs hardening before we rely on it day-to-day.

Tasks

• Session/token lifecycle: refresh expired Bluesky sessions instead of failing the publish
• Surface richer per-provider errors in BlueskyPublishAction / CrosspostPublishAction (rate limits, auth expiry, network) instead of generic failures
• Image/media upload support for Bluesky posts (composer currently only supports an image URL for Instagram)
• Rich text facets: mentions, hashtags, and links should be proper facets, not plain text
• Wire real scheduling: Post/PostTarget models already have status + scheduled_at (with an index), but nothing publishes scheduled posts — add a job/scheduler pass
• Replace the Queue page mock data with real Post records (see UI polish issue)
• Tests for the publish path (driver-level with mocked ATProto responses)

Context

• Driver: app/Services/Social/Drivers/BlueskyDriver.ts, service: app/Services/Social/BlueskyService.ts
• Actions: app/Actions/Postline/Bluesky*.ts
• Fan-out: app/Services/Social/CrosspostService.ts

PR #8 lands most of this: real scheduling (queue + PublishScheduledPosts job), reply-chained Bluesky threads, richer per-provider failure recording, driver-level tests with mocked ATProto responses, and a migration fixing the post_targets provider CHECK that was rejecting Threads inserts. Still open from this list: session refresh hardening beyond the existing withFreshSession path, image/media upload for Bluesky, and rich-text facets.

Shipped across PRs #8, #9, #12, #13: real scheduling with a due-post sweep, reply-chained threads, rich-text facets, image upload (blob + embeds), engagement metrics sync, per-provider failure recording, driver-level tests, and the provider CHECK constraint fix. Remaining nice-to-have (deeper session-refresh hardening beyond withFreshSession) can get its own issue if it ever bites in practice.