docs(prds): social-posting + viral-video PRDs

Two new product specs queued behind the current autoblog + network-gate work:

  docs/social-posting-prd.md
    Buffer/Postiz-style scheduled posting layer. OAuth-first across the
    platforms that actually support content-publish (Reddit, Bluesky,
    Mastodon, LinkedIn, Threads, Pinterest, Tumblr, FB Pages, IG
    Business, YouTube Community, X-paid). Browser-automation fallback
    for Instagram personal / TikTok / Snapchat behind an explicit opt-in
    disclosure, residential proxies, envelope-encrypted credential
    vault (per-user DEK under a KMS-managed KEK — not a single "our
    private key"). 1 credit per platform per post. Tightly integrated
    with autoblog so a new article auto-fans-out as per-platform
    rendered posts. PRD calls out the lawyer-review prerequisite for
    Phase 3.

  docs/viral-video-prd.md
    revid.ai-style short-form vertical video (TikTok / Reels / Shorts).
    7-stage pipeline: Claude script (operator voice, 30-60s) -> OpenAI
    TTS voice -> Pexels b-roll -> royalty-free music -> burn-in
    captions -> Remotion compose -> MP4. v1 COGS ~$0.018/video, healthy
    margin at 1 credit. v2 swaps in ElevenLabs voice; v3 adds AI b-roll
    once prices drop (currently $5-45/video, unviable at 1 credit, PRD
    flags the conflict). Distribution piggybacks on Social Posting,
    explicit dependency. 1 credit per render + N credits per platform
    distributed.

Both intentionally include "open questions" sections so build doesn't
start until the unresolved bits (lawyer review, Remotion vs FFmpeg,
voice library, music licensing) are decided.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ralyodio