Restore native 1080p export on migrated Flow API

[Bl0ck154]

Summary

Restores Flow's native Full HD export on the migrated flow.google.com batchexecute transport.

The new frontend exposes high-resolution download as RPC p0UkFb. This PR captures that wire contract, submits veo_3_1_upsampler_1080p, and resolves the resulting <media-id>_upsampled output through the existing migrated as29s media lookup.

What changes

• add p0UkFb request builder/parser to flow_batch.py
• make FlowClient.upscale_video() work on USE_BATCH_RPC=1
• expose explicit POST /api/flow/export-video with quality: "1080p" as the default
• expose POST /api/flow/check-export-status
• keep /upscale-video / /check-upscale-status as low-level compatibility surfaces
• poll migrated upscale media through as29s, not the dead labs.google REST/tRPC path
• document 1080p as a download/export quality choice rather than an internal implementation detail

Live verification

Verified end-to-end against the current migrated Flow UI/account:

1. Generated a real 720p video through the migrated API.
2. Submitted p0UkFb with veo_3_1_upsampler_1080p.
3. Flow returned <source-media-id>_upsampled.
4. Polled the output through as29s.
5. Downloaded the signed MP4 and checked it with ffprobe.

Result: 1080×1920, H.264, 24 fps, 8.000 s.

The Flow UI currently presents this as 720p original size → 1080p high-resolution download. 4K is plan-gated and was disabled on the verified account, so this PR only claims live verification for 1080p.

Validation

• 88 passed across test_flow_batch.py, test_flow_client_batch.py, and test_upscale_polling.py
• MV3 extension cold-start regression test passes
• git diff --check clean

The branch is based directly on current upstream main (ce37f4b) and intentionally excludes fork-specific browser/session fallback code.