Bound WebRTC upload time by validated progress

[digaobarbosa]

Description

WebRTC uploads can keep workers alive without advancing; only accepted new chunks now refresh the existing idle timeout, while an independent timer enforces the remaining session duration and preserves slow uploads that make progress.

Type of change

• Bug fix

How has this change been tested?

• 31 focused tests pass, including valid MP4 upload with simulated slow transfer, duplicate/ping traffic, expiration, and cancellation cleanup.
• Black, isort, and diff checks pass.
• Staging verification pending.

Any specific deployment considerations

• Stacked on Bound WebRTC video uploads and clean up owned files #2960; merge that dependency first.
• WebRTC workers: Modal CPU/GPU and self-hosted Inference Server.
• Reuses the 60-second default watchdog and session deadline; unlimited local sessions use the existing 3,600-second default for upload duration.
• CI workflows filtering the main base will run after this draft is retargeted.