Add optional ReplayGain analysis to post-processing pipeline

New toggle in Settings → Library → Post-Processing: "Apply ReplayGain
tags after download". When enabled, analyzes loudness via ffmpeg's
ebur128 filter and writes track-level ReplayGain gain/peak tags.
Runs after metadata tagging but before lossy copy so both files get
the tags. Off by default — adds a few seconds per track.

Applied to both album and playlist/single download paths.

web_server.py

@@ -5931,7 +5931,7 @@ def handle_settings():
             if 'active_media_server' in new_settings:
                 config_manager.set_active_media_server(new_settings['active_media_server'])
 
-            for service in ['spotify', 'plex', 'jellyfin', 'navidrome', 'soulseek', 'download_source', 'settings', 'database', 'metadata_enhancement', 'file_organization', 'playlist_sync', 'tidal', 'tidal_download', 'qobuz', 'hifi_download', 'deezer_download', 'lidarr_download', 'listenbrainz', 'acoustid', 'lastfm', 'genius', 'import', 'lossy_copy', 'listening_stats', 'ui_appearance', 'youtube', 'content_filter', 'itunes', 'm3u_export', 'musicbrainz', 'deezer', 'audiodb', 'metadata', 'hydrabase', 'security', 'discogs', 'library', 'discover', 'wishlist', 'genre_whitelist']:
+            for service in ['spotify', 'plex', 'jellyfin', 'navidrome', 'soulseek', 'download_source', 'settings', 'database', 'metadata_enhancement', 'file_organization', 'playlist_sync', 'tidal', 'tidal_download', 'qobuz', 'hifi_download', 'deezer_download', 'lidarr_download', 'listenbrainz', 'acoustid', 'lastfm', 'genius', 'import', 'lossy_copy', 'listening_stats', 'ui_appearance', 'youtube', 'content_filter', 'itunes', 'm3u_export', 'musicbrainz', 'deezer', 'audiodb', 'metadata', 'hydrabase', 'security', 'discogs', 'library', 'discover', 'wishlist', 'genre_whitelist', 'post_processing']:
                 if service in new_settings:
                     for key, value in new_settings[service].items():
                         config_manager.set(f'{service}.{key}', value)
@@ -21346,6 +21346,18 @@ def _post_process_matched_download(context_key, context, file_path):
             # Store final path for verification wrapper (before conversions may override)
             context['_final_processed_path'] = final_path
 
+            # ReplayGain analysis — write track-level gain tags if enabled
+            if config_manager.get('post_processing.replaygain_enabled', False):
+                try:
+                    from core.replaygain import analyze_track as _rg_analyze, write_replaygain_tags as _rg_write, is_ffmpeg_available as _rg_ffmpeg_ok, RG_REFERENCE_LUFS as _RG_REF
+                    if _rg_ffmpeg_ok():
+                        lufs, peak_dbfs = _rg_analyze(final_path)
+                        gain_db = _RG_REF - lufs
+                        _rg_write(final_path, gain_db, peak_dbfs)
+                        pp_logger.info(f"ReplayGain: {gain_db:+.2f} dB — {os.path.basename(final_path)}")
+                except Exception as rg_err:
+                    pp_logger.debug(f"ReplayGain analysis skipped: {rg_err}")
+
             # Downsample hi-res FLAC to CD quality if enabled (must run before lossy copy)
             downsampled_path = _downsample_hires_flac(final_path, context)
             if downsampled_path:
@@ -21723,6 +21735,18 @@ def _post_process_matched_download(context_key, context, file_path):
         # 4. Generate LRC lyrics file at final location (elegant addition)
         _generate_lrc_file(final_path, context, spotify_artist, album_info)
 
+        # 5. ReplayGain analysis — write track-level gain tags if enabled
+        if config_manager.get('post_processing.replaygain_enabled', False):
+            try:
+                from core.replaygain import analyze_track as _rg_analyze, write_replaygain_tags as _rg_write, is_ffmpeg_available as _rg_ffmpeg_ok, RG_REFERENCE_LUFS as _RG_REF
+                if _rg_ffmpeg_ok():
+                    lufs, peak_dbfs = _rg_analyze(final_path)
+                    gain_db = _RG_REF - lufs
+                    _rg_write(final_path, gain_db, peak_dbfs)
+                    pp_logger.info(f"ReplayGain: {gain_db:+.2f} dB, peak {peak_dbfs:.2f} dBFS — {os.path.basename(final_path)}")
+            except Exception as rg_err:
+                pp_logger.debug(f"ReplayGain analysis skipped: {rg_err}")
+
         # Downsample hi-res FLAC to CD quality if enabled (must run before lossy copy)
         downsampled_path = _downsample_hires_flac(final_path, context)
         if downsampled_path:

webui/index.html

@@ -5195,6 +5195,13 @@
                                                 Generate .lrc lyrics files (LRClib)
                                             </label>
                                         </div>
+                                        <div class="form-group">
+                                            <label class="checkbox-label">
+                                                <input type="checkbox" id="replaygain-enabled">
+                                                Apply ReplayGain tags after download
+                                            </label>
+                                            <small class="settings-hint">Analyzes loudness and writes ReplayGain track gain/peak tags. Requires ffmpeg. Adds a few seconds per track.</small>
+                                        </div>
                                     </div>
 
                                     <!-- Tag Embedding — per-tag toggles grouped by source -->

webui/static/script.js

@@ -6123,6 +6123,7 @@ async function loadSettingsData() {
         document.getElementById('cover-art-download').checked = settings.metadata_enhancement?.cover_art_download !== false;
         document.getElementById('prefer-caa-art').checked = settings.metadata_enhancement?.prefer_caa_art === true;
         document.getElementById('lrclib-enabled').checked = settings.metadata_enhancement?.lrclib_enabled !== false;
+        document.getElementById('replaygain-enabled').checked = settings.post_processing?.replaygain_enabled === true;
         // Load service master toggles
         document.getElementById('embed-spotify').checked = settings.spotify?.embed_tags !== false;
         document.getElementById('embed-itunes').checked = settings.itunes?.embed_tags !== false;
@@ -7456,6 +7457,9 @@ async function saveSettings(quiet = false) {
             enabled: document.getElementById('genre-whitelist-enabled').checked,
             genres: _collectGenreWhitelist(),
         },
+        post_processing: {
+            replaygain_enabled: document.getElementById('replaygain-enabled').checked,
+        },
         library: {
             music_paths: collectMusicPaths(),
             music_videos_path: document.getElementById('music-videos-path').value || './MusicVideos'