fix(audio): make the idle level meter follow the operator's audio-source pick

[andrescera]

What

The Live page's Audio source selection now drives the idle audio-level meter.
config.asrc is resolved to an ALSA device and pushed to cerastream as the additive
reload-config → audio.meter_device field (engine schema 0.9.0).

Backend-only. No frontend change: the picker, the meter component and the wire payload
they exchange are all untouched — the meter simply starts reporting the card the picker
is already showing.

Depends on CERALIVE/cerastream#72
for the engine half, but does NOT need it merged to land safely: the send is gated on the
engine advertising schema ≥ 0.9.0, so against today's engine this PR is a no-op.

Why

Found live during board QA. The operator selected the RØDE HDMI-to-USB-C, SourceSection
showed the RØDE, and the meter reported the DJI Mic Mini — or "Meter unavailable". The
engine picked its own idle card with no input from CeraUI, so with two healthy cards the
one whose vendor string sorts first simply won. Selecting an audio source changed nothing
about the meter, and there was no way for the operator to correct it.

How

• resolveMeterPreference(asrc) (modules/streaming/audio.ts) turns the picker value
  into the device the meter should prefer, or null for "engine, choose for yourself".
  It reuses the SAME audioDevices map, alias reverse-lookup and hw:CARD= wrapping that
  resolveAudioMode already uses for start, so the meter and the program leg can never
  disagree about which card a pick names. It is deliberately NOT resolveAudioMode: this
  is the IDLE meter, it has no notion of network-embedded program audio, and it must keep
  following the card the picker is showing. null covers AUDIO_SOURCE_AUTO, both
  pipeline pseudo-sources, an unset asrc, and anything that resolves to no card.
• The audio-meter-bridge delivers it, because it already holds the ONE long-lived
  IDLE connection to the engine — cerastream-backend's client only exists while
  streaming. It pushes on every connect (the engine keeps no preference across a restart),
  and syncAudioMeterPreference() re-pushes from streaming.setConfig when asrc changed
  and from updateAudioDevices, since a re-enumeration can change which card an
  unchanged pick resolves to.
• reload-config, not switch-audio. switch-audio is stream-only and answers
  not_streaming exactly when the idle meter is running. reload-config already carries
  an audio section and is idle-safe on the engine side.
• rawRequest behind supportsMeterDevicePreference. The published
  @ceralive/cerastream client Zod-strips the additive meter_device key, so the typed
  reloadConfig() would silently drop it — the same constraint audio.mode and
  video_passthrough already work around. The gate is fail-safe false, so a pre-0.9.0
  engine is sent nothing and keeps auto-picking: exactly its current behaviour.
• Absent ≠ null. Absent leaves the engine's preference unchanged (so reloadAudioDelay's
  delay-only reload can never clear it); null means Auto. Not interchangeable.
• A refused push never breaks the meter — logged and swallowed, the previous preference
  stands, and the next config change or reconnect re-pushes.

It is a preference, not a pin, and the engine guarantees that. cerastream only moves
the named card to the head of its candidate list; its delivery-confirmation demotion
(PR #71) is unchanged, so selecting a powered-off receiver still lands on a working card
rather than a dead meter. There is deliberately no CeraUI-side "force this device" path.

How to verify

Unit — bun test in apps/backend (2256 pass):

• tests/audio-meter-bridge.test.ts — pushes the selected card on connect; "Auto" sends
  an explicit null; re-pushes after a source change; sends NOTHING to a pre-0.9.0 engine
  (and levels keep flowing); a refused reload leaves the meter running; syncing while down
  is a silent no-op; plus the schema gate table.
• tests/audio-sources.test.ts — resolveMeterPreference: alias, no-alias, every null
  case, and selector passthrough.

The three push assertions were confirmed RED against the un-wired bridge before the push
landed; the ten pre-existing bridge tests stayed green throughout.

On real hardware — Rock 5B+ 192.168.78.131, RØDE HDMI-to-USB-C attached, operator
config already at asrc: "USB audio" (untouched):

audio-meter bridge: subscribed to the engine audio-level topic
audio-meter bridge: idle-meter preference set to hw:CARD=usbaudio

and the meter then reported 41 live level events over 8 s, all card:usbaudio, 0
unavailable, with real moving audio (rms_db ≈ -31…-36, not the digital-silence floor).
The updateAudioDevices re-push path was exercised live via SIGUSR2. A preference naming a
card the engine does not enumerate was verified inert — the meter stayed on the RØDE and
never went unavailable.

Gates: bun run lint (exit 0), apps/backend bun test 2256 pass / 0 fail,
tsc --noEmit clean, check:tech-debt OK.

Two local pre-push lanes could not run here and are NOT regressions: test-fe fails
identically on a clean origin/main checkout (a local jsdom localStorage quirk — all
1319 frontend tests themselves pass), and the four e2e lanes fail at
playwright install-deps, the documented can't-run-locally caveat in docs/LOCAL-CI.md.
Cloud CI is the authority for both.

Risks

• Ordering with cerastream#72. Until it merges and ships, the schema gate keeps this
  PR inert — the engine advertises 0.8.0 and is sent nothing. No flag day.
• rawRequest bypasses the typed client. Deliberate and pre-existing practice
  (switch-audio, start with audio.mode); the payload is a two-key object and the
  engine validates it.
• One extra reload-config per connect and per audio re-enumeration. Cheap, idle-safe,
  and non-blocking; a failure cannot stall the meter.
• The diff is backend-only, so no visual or interaction change to review.

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