feat(audio): channel sample_rate caps Opus bandwidth (narrowband/wideband)

The per-channel sample_rate field was inert after pinning the codec to
48 kHz. Make it meaningful without changing the 48 kHz clock: carry it as
OpusParams::max_bandwidth_hz and apply OPUS_SET_MAX_BANDWIDTH in
OpusEncoder::init (8000->narrowband, 16000->wideband, 24000->super-wideband,
48000->full). A low-bitrate room can now shed out-of-band content while
every endpoint keeps a single 48 kHz clock.

Make sample_rate channel-authoritative on the server: conn_session no
longer overrides effective sample_rate with the client's always-48000
request (it now behaves like frame_ms/mode). vc_get_stream_audio_config
reports the channel's configured rate for own streams too.

New ctest channel_samplerate: a 7 kHz tone is attenuated ~1000x on an
8 kHz (narrowband) channel vs a 48 kHz (full-band) channel, proving the cap
is in effect. ctest --preset dev 26/26.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -8,7 +8,7 @@ and what's next* read [`PROGRESS.md`](PROGRESS.md); for *design* read [`docs/`](
 > server-mute, channel CRUD, in-app account management, disconnect/keepalive/reaper. Windows
 > WinForms C# client shipped (M4). **macOS AppKit client shipped** — `VoiceCatMac.xcodeproj`
 > at `clients/apple/macOS/`. **iOS SwiftUI client shipped** — `VoiceCatiOS.xcodeproj` at
-> `clients/apple/iOS/`. `ctest --preset dev` green — 25/25 tests.
+> `clients/apple/iOS/`. `ctest --preset dev` green — 26/26 tests.
 > External PCM feed/tap API (`vc_stream_feed_pcm` + `vc_set_pcm_sink`) shipped.
 > **Screen-audio sharing shipped on macOS (ScreenCaptureKit) and iOS (ReplayKit Broadcast
 > Upload Extension → host App Group ring → `vc_stream_feed_pcm`).** See [`PROGRESS.md`](PROGRESS.md).