feat: publishing engine improvements — rate limit retry, inline token refresh, per-platform queues, proactive refresh
- Add HasSocialHttpClient trait with 429 rate limit retry (3 attempts, 5s delay)
- Integrate trait into all 10 publishers (YouTube uses Google SDK)
- Add inline token refresh retry in PublishToSocialPlatform job
- Add per-platform Horizon queues via Platform::queue() and Platform::allQueues()
- Add RefreshExpiringTokens hourly command for proactive token refresh
- Fix token leaks: redact response bodies in all Log::error calls
- Fix token leaks: remove $response->body() from exception messages
- Fix ConnectionVerifier: redact all refresh error logs
- Fix null checks on API response IDs (Instagram, Threads, Pinterest, Facebook)
- Fix PublishPost::failed() to mark post as failed
- Fix StoreChunkedMediaRequest: validate max 1GB total size
- Fix scheduled_at validation: string → date
- Fix StoreMediaRequest: images max 10MB, videos max 1GB, only MP4 video
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