feat(api): library sources, scanner and catalog (M2)

- schema for sources, path patterns, media files, artists, albums,
  tracks, genres (migration 00003)
- path patterns (RE2 with named groups, skip), merge with tags by
  source priority, fallbacks from the file name and folder, name
  normalization and multi-artist split
- scanner: walk, skip unchanged files by size and mtime, ffprobe in 8
  workers with a single catalog writer, renames by content hash keep
  the track, duplicates fold into one track with the best file as
  primary, missing files are kept 30 days
- storage-agent mounts NFS with validated server, export and an option
  whitelist, no shell; test mount counts audio files
- admin API: sources CRUD, connection test, patterns with preview,
  scan, library status, file errors, host folder picker; scheduled
  scans and health checks; mounts are restored on start
- catalog API: artist, artist albums, album with tracks, track
- api image is alpine with ffmpeg for ffprobe

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/data-model.md

@@ -197,7 +197,7 @@ CREATE TABLE albums (
   id               uuid PRIMARY KEY,
   title            text NOT NULL,
   title_norm       text NOT NULL,
-  album_artist_id  uuid REFERENCES artists(id),
+  album_artist_id  uuid NOT NULL REFERENCES artists(id),  -- без тегов — исполнитель трека или «Unknown artist»
   year             int,
   cover_path       text,                 -- путь в кэше обложек (/cache/covers/<hash>.webp)
   cover_colors     text[],               -- палитра обложки (3 цвета: доминирующий, второй, акцент) — для ambient-фона,
@@ -208,7 +208,8 @@ CREATE TABLE albums (
 CREATE TABLE tracks (
   id            uuid PRIMARY KEY,
   title         text NOT NULL,
-  album_id      uuid REFERENCES albums(id),
+  title_norm    text NOT NULL,                -- ключ поиска дублей: (album_id, title_norm, duration ±2 с)
+  album_id      uuid NOT NULL REFERENCES albums(id),
   disc_no       int, track_no int,
   year          int,
   duration_ms   int NOT NULL,