[Rooms][P1] Productionize WebRTC with TURN, quality metrics, and an SFU path

[knzeng-e]

Outcome

Prove that shared listening survives restrictive real networks and define the measured boundary between mesh and SFU delivery.

Delivered

• Resume/rejoin states, deterministic host-disconnect handling, listener caps, and bounded room-quality telemetry.
• API-issued short-lived TURN REST credentials; the public grant path is configured.
• Host-only protected key custody, walletless guests, and explicit degraded states.

Remaining acceptance

• Force relay-only media and verify allocation from two independent networks.
• Capture join-to-connected, first remote audio, loss, jitter, RTT, reconnect, and relay/direct evidence.
• Exercise iPhone background/foreground and network-loss recovery.
• Measure host upstream/CPU and keep conservative caps until evidence justifies change.
• Record when mesh remains acceptable and when an SFU becomes mandatory.

Run this as part of W13. Do not start an SFU migration before the measurements establish the need.

Local scope: docs/backlog/polkadot-product-readiness-and-killer-dapp-roadmap.md and docs/backlog/implementation/evidence/W09.md.

[knzeng-e]

Backlog sync 2026-07-14: keep this focused on TURN/SFU/reconnect and room reliability. Statement Store is limited to small signed ephemeral presence/discovery until its signer, TTL, and payload limits are solved; do not move SDP/ICE, durable chat history, media metadata, or walletless guest reactions there in this ticket.

[knzeng-e]

Deploy-track objective after #137 merge: keep #89 open and use the Product executable [0, 1, 13] deploy to collect real shared-listening evidence. Next evidence gate: host opens a room in Product DevNet, ordinary-browser guest joins from the canonical link without wallet friction, guest can recover blocked autoplay with Start audio, and both sides capture room-quality telemetry (window.__DOTIFY_ROOM_QUALITY__.snapshot()). TURN/forced-relay and SFU/cap decisions remain out of scope until measured evidence exists.