Add MusicBrainz barcode lookup (scan --barcode and interactive rip)

- New module musicbrainz.py: lookup_by_barcode() via EAN-13/UPC-12,
  two-step API (barcode search → release detail with recordings),
  respects 1 req/s rate limit with User-Agent header
- cli.py: scan command gets --barcode option as highest-priority mode
  (no images needed); _scan_to_album() dispatches to MusicBrainz first
- ripper.py: interactive_rip() prompts for optional EAN after album name;
  MusicBrainz data (incl. year) takes priority over CDDB for album.json;
  album_root.mkdir() added so JSON can be written even when MB changes dir
- tests: test_musicbrainz.py (16 tests), test_ripper.py +6 barcode tests

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/musiksammlung/musicbrainz.py

@@ -0,0 +1,110 @@
+"""MusicBrainz-Lookup via EAN/Barcode."""
+
+from __future__ import annotations
+
+import logging
+import time
+
+import httpx
+
+from musiksammlung.models import Album, Disc, Track
+
+logger = logging.getLogger(__name__)
+
+_MB_BASE = "https://musicbrainz.org/ws/2"
+_USER_AGENT = "musiksammlung/0.1 ( https://kitux.de/forgejo/dschlueter/Musiksammlung )"
+_RATE_SLEEP = 1.1  # MusicBrainz erlaubt max. 1 Request/Sekunde
+
+
+def _get(path: str, params: dict) -> dict:
+    """HTTP-GET gegen die MusicBrainz-API mit korrektem User-Agent."""
+    response = httpx.get(
+        f"{_MB_BASE}{path}",
+        params=params,
+        headers={"User-Agent": _USER_AGENT},
+        timeout=30.0,
+    )
+    response.raise_for_status()
+    return response.json()
+
+
+def lookup_by_barcode(ean: str) -> Album:
+    """Schlägt ein Album anhand des EAN-Barcodes in MusicBrainz nach.
+
+    Führt zwei API-Requests durch:
+      1. Barcode-Suche → MBID des ersten Treffers
+      2. Release-Details mit Recordings → Trackliste
+
+    Args:
+        ean: EAN-13- oder UPC-12-Barcode
+
+    Returns:
+        Album mit vollständiger Trackliste
+
+    Raises:
+        ValueError: Kein Eintrag für diesen Barcode gefunden
+        httpx.HTTPError: Netzwerk- oder API-Fehler
+    """
+    # Schritt 1: Barcode-Suche
+    logger.info("MusicBrainz: Suche nach Barcode %s", ean)
+    data = _get("/release/", {"query": f"barcode:{ean}", "fmt": "json"})
+
+    releases = data.get("releases", [])
+    if not releases:
+        raise ValueError(f"Kein MusicBrainz-Eintrag für Barcode {ean!r} gefunden.")
+
+    mbid = releases[0]["id"]
+    logger.info("MusicBrainz: Treffer MBID=%s, lade Details...", mbid)
+    time.sleep(_RATE_SLEEP)
+
+    # Schritt 2: Trackliste laden
+    detail = _get(f"/release/{mbid}", {"inc": "recordings", "fmt": "json"})
+    return _parse_release(detail)
+
+
+def _parse_release(data: dict) -> Album:
+    """Wandelt eine MusicBrainz-Release-Antwort in ein Album-Modell um."""
+    # Künstler
+    artist_credit = data.get("artist-credit", [])
+    artist = artist_credit[0]["artist"]["name"] if artist_credit else ""
+
+    # Albumtitel
+    title = data.get("title", "")
+
+    # Jahr aus "date" extrahieren ("YYYY", "YYYY-MM" oder "YYYY-MM-DD")
+    year: int | None = None
+    date_str = data.get("date", "")
+    if date_str and len(date_str) >= 4:
+        try:
+            year = int(date_str[:4])
+        except ValueError:
+            pass
+
+    # Medien → Discs
+    discs: list[Disc] = []
+    for medium in data.get("media", []):
+        disc_number = medium.get("position", len(discs) + 1)
+        tracks: list[Track] = []
+        for t in medium.get("tracks", []):
+            track_number = t.get("position", len(tracks) + 1)
+            track_title = t.get("title", "")
+
+            # Track-Künstler nur setzen, wenn er vom Album-Künstler abweicht
+            t_credits = t.get("artist-credit", [])
+            track_artist: str | None = None
+            if t_credits:
+                t_artist = t_credits[0]["artist"]["name"]
+                if t_artist != artist:
+                    track_artist = t_artist
+
+            tracks.append(Track(
+                track_number=track_number,
+                title=track_title,
+                artist=track_artist,
+            ))
+        discs.append(Disc(disc_number=disc_number, tracks=tracks))
+
+    if not discs:
+        raise ValueError("MusicBrainz-Release enthält keine Medien/Tracks.")
+
+    return Album(artist=artist, album=title, year=year, discs=discs)