feat: add quality/transcoding options to live player (#6)

Manual quality profiles (source/1080p/720p/480p) in the player overflow
menu plus an honest/preparatory Auto contract: adaptive when the HLS
manifest exposes multiple levels, single-rendition automatic selection
otherwise.

- server: transcodeQualityProfiles service and qualityMode session option
- player: quality menu, persisted preference, live switch with rollback
- tests: npm test script and unit tests for quality profiles

test/transcodeQualityProfiles.test.js

@@ -0,0 +1,82 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+
+const {
+    QUALITY_MODES,
+    resolveQualityMode
+} = require('../server/services/transcodeQualityProfiles');
+const { TranscodeSession } = require('../server/services/transcodeSession');
+
+test('legacy payloads do not receive profile overrides', () => {
+    assert.equal(resolveQualityMode(undefined, { maxResolution: '720p' }), null);
+});
+
+test('manual quality profiles map to encoding height and bitrate', () => {
+    const expected = {
+        '1080p': { maxResolution: '1080p', targetVideoBitrateKbps: 6000 },
+        '720p': { maxResolution: '720p', targetVideoBitrateKbps: 3500 },
+        '480p': { maxResolution: '480p', targetVideoBitrateKbps: 1500 }
+    };
+
+    for (const [qualityMode, profile] of Object.entries(expected)) {
+        const selection = resolveQualityMode(qualityMode);
+        assert.equal(selection.mode, 'single');
+        assert.equal(selection.selectedProfile, qualityMode);
+        assert.deepEqual(selection.availableProfiles, QUALITY_MODES);
+        assert.deepEqual(selection.sessionOptions, {
+            videoMode: 'encode',
+            ...profile,
+            upscaleEnabled: false
+        });
+    }
+});
+
+test('profile options produce the expected FFmpeg scale and bitrate arguments', () => {
+    for (const qualityMode of ['1080p', '720p', '480p']) {
+        const selection = resolveQualityMode(qualityMode);
+        const session = new TranscodeSession('https://example.test/live.ts', {
+            hwEncoder: 'software',
+            audioMixPreset: 'passthrough',
+            ...selection.sessionOptions
+        });
+        const args = session.buildFFmpegArgs('software');
+        const bitrate = selection.sessionOptions.targetVideoBitrateKbps;
+
+        assert.equal(args[args.indexOf('-vf') + 1], `scale=-2:${qualityMode.replace('p', '')}`);
+        assert.equal(args[args.indexOf('-b:v') + 1], `${bitrate}k`);
+        assert.equal(args[args.indexOf('-maxrate') + 1], `${bitrate}k`);
+        assert.equal(args[args.indexOf('-bufsize') + 1], `${bitrate * 2}k`);
+    }
+});
+
+test('source profile preserves the original video stream', () => {
+    const selection = resolveQualityMode('source');
+
+    assert.equal(selection.selectedProfile, 'source');
+    assert.deepEqual(selection.sessionOptions, { videoMode: 'copy' });
+});
+
+test('auto selects one configured rendition and reports single-rendition mode', () => {
+    const selection = resolveQualityMode('auto', { maxResolution: '720p' });
+
+    assert.equal(selection.qualityMode, 'auto');
+    assert.equal(selection.mode, 'single');
+    assert.equal(selection.selectedProfile, '720p');
+    assert.deepEqual(selection.sessionOptions, {
+        videoMode: 'encode',
+        maxResolution: '720p',
+        targetVideoBitrateKbps: 3500,
+        upscaleEnabled: false
+    });
+});
+
+test('auto falls back to 1080p for an unsupported global resolution', () => {
+    assert.equal(resolveQualityMode('auto', { maxResolution: '4k' }).selectedProfile, '1080p');
+});
+
+test('unsupported quality modes are rejected', () => {
+    assert.throws(
+        () => resolveQualityMode('adaptive'),
+        error => error.code === 'INVALID_QUALITY_MODE'
+    );
+});