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

server/routes/transcode.js

@@ -5,6 +5,7 @@ const path = require('path');
 const fs = require('fs').promises;
 const db = require('../db');
 const transcodeSession = require('../services/transcodeSession');
+const { QUALITY_MODES, resolveQualityMode } = require('../services/transcodeQualityProfiles');
 
 /**
  * Transcode Routes
@@ -31,7 +32,8 @@ transcodeSession.startCleanupInterval();
 router.post('/session', async (req, res) => {
     const {
         url, seekOffset, videoMode, videoCodec, audioCodec, audioChannels,
-        targetVideoBitrateKbps, targetAudioBitrateKbps, allowHardwareFallback
+        targetVideoBitrateKbps, targetAudioBitrateKbps, allowHardwareFallback,
+        qualityMode
     } = req.body;
 
     if (!url) {
@@ -43,6 +45,7 @@ router.post('/session', async (req, res) => {
     const userAgent = db.getUserAgent(settings);
 
     try {
+        const qualitySelection = resolveQualityMode(qualityMode, settings);
         const session = await transcodeSession.createSession(url, {
             ffmpegPath,
             userAgent,
@@ -61,7 +64,8 @@ router.post('/session', async (req, res) => {
             videoMode: videoMode, // 'copy' or 'encode'
             videoCodec: videoCodec, // 'h264', 'hevc', etc.
             audioCodec: audioCodec, // 'aac', 'ac3', etc.
-            audioChannels: audioChannels // number of channels (2=stereo)
+            audioChannels: audioChannels, // number of channels (2=stereo)
+            ...(qualitySelection?.sessionOptions || {})
         });
 
         await session.start();
@@ -80,10 +84,22 @@ router.post('/session', async (req, res) => {
             status: session.status,
             requestedEncoder: session.requestedEncoder,
             effectiveEncoder: session.effectiveEncoder,
-            hardwareFallbackOccurred: session.hardwareFallbackOccurred
+            hardwareFallbackOccurred: session.hardwareFallbackOccurred,
+            ...(qualitySelection && {
+                qualityMode: qualitySelection.qualityMode,
+                mode: qualitySelection.mode,
+                selectedProfile: qualitySelection.selectedProfile,
+                availableProfiles: qualitySelection.availableProfiles
+            })
         });
 
     } catch (err) {
+        if (err.code === 'INVALID_QUALITY_MODE') {
+            return res.status(400).json({
+                error: err.message,
+                availableProfiles: QUALITY_MODES
+            });
+        }
         console.error('[Transcode] Session creation failed:', err);
         res.status(500).json({ error: 'Failed to create session', details: err.message });
     }