Community chat: Stream Chat integration (free ≤1k MAU) with moderation + cost tripwire

[teetangh]

Adds real-time channels alongside in-house discussions (kept as archive). Stream Chat free ≤1,000 MAU / 100 concurrent; paid cliff $399/mo at 1K MAU.

• src/lib/stream/chat.ts token minting (role mapping, entitled learners only)
• /learn/community channels: per-course + platform-wide; discussions preserved as archive tab
• Moderation: Stream moderation config; admin/moderator roles; flags → IntegrityFlag(MESSAGE) → /admin moderation
• DPDP: Mumbai data region request, retention policy, device-token cleanup on sign-out
• Nightly MAU tripwire alert before billing cliff

Spec: temp prompt library 05-chat-community.