feat(liveion): source quality tiers and manual bitrate switching

[a-wing]

Background

Main already has the network-driven adaptive bitrate loop (issue #409): WHEP subscriber RTCP feedback (TWCC / RR loss) is sampled per subscriber, and a per-stream AIMD controller retunes the shared hardware encoder through StreamSource::set_bitrate → livehal EncoderBackend::setBitrate. Until now only the rkmpp backend implemented runtime retuning, and there was no way to drive the encoder bitrate by hand.

What this PR adds

livehal: v4l2-m2m runtime retune

• V4l2M2mEncoder::setBitrate() — VIDIOC_S_CTRL / V4L2_CID_MPEG_VIDEO_BITRATE on the running device, joining rkmpp. RDK X5 stays unsupported (needs on-device verification).

Config: named quality tiers at source level

[[stream.pi-cam.sources.tiers]]
name = "low"
bitrate = 600000

• Tiers sit next to capture/encoder (not inside the encoder block) because a quality tier spans both: bitrate retunes the encoder, while resolution/framerate tiers would also reconfigure the capture (livehal has no scaler stage). Reserved per-tier width/height/fps fields exist in the schema but are rejected by validation until the encoder-rebuild path lands — today tiers are bitrate-only presets, validated unique, non-zero, and ≤ encoder.bitrate.
• When tiers are declared, the AIMD floor defaults to the lowest tier (explicit min_bitrate still wins). Old configs are unaffected.

API: /api/sources/{stream}/bitrate

• GET — drive mode (adaptive/manual/fixed), current bitrate, active override and tier match, configured tiers.
• POST — { "tier": "mid" } or { "bitrate": 1500000 } (exactly one; 400 on both/neither/unknown tier, 404 without a source, 409 when the backend cannot retune).
• DELETE — clears the manual override; the AIMD controller resumes from the held value, fixed sources just drop the marker.
• A per-stream BitrateControl is now created for every native source (not only adaptive ones) so manual/auto coordination and state reporting behave uniformly.

WebUI

• Per-stream Bitrate button in the streams table opens a dialog (lazy GET): mode badge + current bitrate, tier buttons with active highlight, raw bps input, resume/clear-override action, graceful empty state for streams without an encoder source (also covers liveman, which does not proxy /api/sources).

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --features source-all,webui,net4mqtt,recorder,cascade,whepwright,rtsp,target-whip,native-source,capture-v4l2,encoder-v4l2-m2m -- -D warnings
• cargo test -p liveion ... --lib — 216 passed (11 new: tier validation, TOML tiers roundtrip, tier resolution, bitrate-info modes)
• cargo test --features source --test tests — incl. extended bitrate-API smoke test (400/404 paths)
• pnpm run lint, pnpm run typecheck, pnpm -r build
• Hardware end-to-end (POST actually retuning a live encoder) needs a V4L2/MPP device — not verifiable on the dev machine; to be exercised on RK3588 / RPi.

[codecov]

Codecov Report

❌ Patch coverage is 47.78761% with 177 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
liveion/src/route/source.rs	4.80%	99 Missing ⚠️
liveion/src/stream/source/manager.rs	65.71%	48 Missing ⚠️
liveion/src/error.rs	0.00%	18 Missing ⚠️
liveion/src/stream/source/adaptive_bitrate.rs	86.15%	9 Missing ⚠️
liveion/src/stream/source/mod.rs	50.00%	3 Missing ⚠️

Files with missing lines	Coverage Δ
liveion/src/config.rs	94.45% <ø> (ø)
liveion/src/stream/manager.rs	82.72% <100.00%> (+0.07%)	⬆️
liveion/src/stream/source/mod.rs	84.48% <50.00%> (-1.24%)	⬇️
liveion/src/stream/source/adaptive_bitrate.rs	80.00% <86.15%> (+2.42%)	⬆️
liveion/src/error.rs	47.36% <0.00%> (-21.87%)	⬇️
liveion/src/stream/source/manager.rs	73.56% <65.71%> (-4.30%)	⬇️
liveion/src/route/source.rs	6.30% <4.80%> (-1.17%)	⬇️

... and 3 files with indirect coverage changes

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