feat(library): lazy materialization foundation for remote tracks (§Phase1)

Adds nullable storage fields + availability column on tracks, remote
source/source_id identity on albums/artists, TrackRepository.materialize()
and get_or_create_remote() repos — groundwork for on-demand YTM library
(placeholders saved without audio, materialized in-place on first play).

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

app/infrastructure/db/models/album.py

@@ -2,7 +2,7 @@
 
 import uuid
 
-from sqlalchemy import ForeignKey, Integer, String
+from sqlalchemy import ForeignKey, Integer, String, UniqueConstraint
 from sqlalchemy.orm import Mapped, mapped_column
 
 from app.infrastructure.db.base import Base
@@ -11,6 +11,12 @@ from app.infrastructure.db.models.mixins import TimestampMixin, UUIDPrimaryKeyMi
 
 class AlbumModel(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     __tablename__ = "albums"
+    __table_args__ = (
+        # Binds a remote (browsable) album to its local row for re-browse/save
+        # dedup. Multiple NULLs are allowed by Postgres, so locally-created
+        # albums (source/source_id both NULL) never collide on this.
+        UniqueConstraint("source", "source_id", name="uq_albums_source_source_id"),
+    )
 
     title: Mapped[str] = mapped_column(String(1024), index=True, nullable=False)
     artist_id: Mapped[uuid.UUID] = mapped_column(
@@ -21,3 +27,7 @@ class AlbumModel(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     year: Mapped[int | None] = mapped_column(Integer, nullable=True)
     cover_path: Mapped[str | None] = mapped_column(String(1024), nullable=True)
     musicbrainz_id: Mapped[str | None] = mapped_column(String(36), index=True, nullable=True)
+
+    # -- remote identity (lazy materialization) --------------------------
+    source: Mapped[str | None] = mapped_column(String(32), nullable=True)
+    source_id: Mapped[str | None] = mapped_column(String(512), nullable=True)