[EMB-123] AI Feed

[chandlerhuff]

Summary

Documents the new per-subscription AI Atom feed feature: users can now enable a private AI Feed on any subscription, configure its lookback window and similarity threshold, and optionally enable Exa web retrieval. Each subscription publishes a signed Atom URL that serves AI summaries and related articles without calling the external AI services at request time.

Changes

• Added a Configure AI Feed step to the Subscriptions workflow, including the 48-hour default lookback, per-subscription Exa tuning, regeneration behavior on settings change, and the empty-feed failure state (missing credentials, unavailable model, pending retries).
• Documented the new GET /api/feeds/ai/{settingId} route in the HTTP API reference: default 100 entries, limit from 1 to 1000, 404 for disabled feeds, 401 for changed scope or signature, and confirmation that feed requests use Qdrant only (never Exa or OpenCode).
• Added AI_FEED_SIGNING_SECRET and AI_FEED_ENCRYPTION_KEY to the environment variables reference, including their fallback behavior to NEXTAUTH_SECRET.
• Noted that AI Feed URLs are credentials — their signatures grant read access and must be handled like secrets.

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Ticket: EMB-123