Live classes: fix #53 join-fallback + add Stream Video provider

[teetangh]

Founder decision: standardize on Stream (getstream.io) for video AND chat (vendor consistency). Stream livestream call type covers host→cohort; $100/mo credit covers beta usage (~$0–110/mo).

Phase 1 — #53 (ship standalone, vendor-independent)

• Provisioning failure → Sentry warning + flagged session; learner sees explicit 'setup in progress'; creator retry/paste-URL action flipping provider to EXTERNAL_LINK
• Regression test: rejected provisioning still creates visible session

Phase 2 — STREAM provider

• LiveProvider.STREAM enum (⚠️ schema-approved label) + env STREAM_API_KEY/SECRET
• src/lib/stream/live.ts: create livestream call, mint host/viewer tokens, recordings fetch
• Schedule dialog provider select; learner embedded join 15-min-before; recording → VideoAsset
• Cost metric logging per session (participant-minutes)

Specs: temp prompt library 04-live-classes-stream. Related #53.