feat: Add cross-band entity relationship models

Schema additions for fediversion multi-band architecture:

- SongCanon: Canonical master songs for cross-band linking
  (e.g., 'Dark Star' links GD, D&C, Billy Strings versions)

- UserVerticalPreference: User band display preferences
  - display_mode: primary/secondary/attribution_only/hidden
  - priority: Sort order for UI
  - notify_on_show: Per-band notification settings

- Vertical additions:
  - primary_artist_id: Link to main Artist entity
  - color: Hex branding color
  - emoji: Display emoji

- Song additions:
  - canon_id: Link to SongCanon for cross-band tracking

backend/models.py

@@ -61,6 +61,13 @@ class Vertical(SQLModel, table=True):
     slug: str = Field(unique=True, index=True)
     description: Optional[str] = Field(default=None)
     
+    # Link to primary artist/band for this vertical
+    primary_artist_id: Optional[int] = Field(default=None, foreign_key="artist.id")
+    
+    # Theming
+    color: Optional[str] = Field(default=None, description="Hex color for branding")
+    emoji: Optional[str] = Field(default=None, description="Display emoji")
+    
     shows: List["Show"] = Relationship(back_populates="vertical")
     songs: List["Song"] = Relationship(back_populates="vertical")
 
@@ -155,6 +162,18 @@ class Show(SQLModel, table=True):
     attendances: List["Attendance"] = Relationship(back_populates="show")
     performances: List["Performance"] = Relationship(back_populates="show")
 
+class SongCanon(SQLModel, table=True):
+    """Canonical 'master' song independent of band - enables cross-band song linking"""
+    id: Optional[int] = Field(default=None, primary_key=True)
+    title: str = Field(index=True)
+    slug: str = Field(unique=True, index=True)
+    original_artist: Optional[str] = Field(default=None, description="Original songwriter/band")
+    original_artist_id: Optional[int] = Field(default=None, foreign_key="artist.id")
+    notes: Optional[str] = Field(default=None)
+    
+    # All vertical-specific versions of this song
+    versions: List["Song"] = Relationship(back_populates="canon")
+
 class Song(SQLModel, table=True):
     id: Optional[int] = Field(default=None, primary_key=True)
     title: str = Field(index=True)
@@ -164,11 +183,15 @@ class Song(SQLModel, table=True):
     notes: Optional[str] = Field(default=None)
     youtube_link: Optional[str] = Field(default=None)
     
-    # New Relation
-    artist_id: Optional[int] = Field(default=None, foreign_key="artist.id")
-    artist: Optional[Artist] = Relationship(back_populates="songs")
+    # Link to canonical song for cross-band tracking
+    canon_id: Optional[int] = Field(default=None, foreign_key="songcanon.id")
+    canon: Optional[SongCanon] = Relationship(back_populates="versions")
     
-    vertical: Vertical = Relationship(back_populates="songs")
+    # Artist who wrote/performs this version
+    artist_id: Optional[int] = Field(default=None, foreign_key="artist.id")
+    artist: Optional["Artist"] = Relationship(back_populates="songs")
+    
+    vertical: "Vertical" = Relationship(back_populates="songs")
 
 class Sequence(SQLModel, table=True):
     """Named groupings of consecutive songs, e.g. 'Autumn Crossing' = Travelers > Elmeg the Wise"""
@@ -346,7 +369,20 @@ class UserPreferences(SQLModel, table=True):
     email_on_chase: bool = Field(default=True, description="Email when your chase song is played")
     email_digest: bool = Field(default=False, description="Weekly digest email")
     
-    user: User = Relationship(back_populates="preferences")
+    user: "User" = Relationship(back_populates="preferences")
+
+class UserVerticalPreference(SQLModel, table=True):
+    """User preferences for which bands to display prominently vs. attribution-only"""
+    id: Optional[int] = Field(default=None, primary_key=True)
+    user_id: int = Field(foreign_key="user.id", index=True)
+    vertical_id: int = Field(foreign_key="vertical.id", index=True)
+    display_mode: str = Field(default="primary", description="primary, secondary, attribution_only, hidden")
+    priority: int = Field(default=0, description="Sort order - lower = higher priority")
+    notify_on_show: bool = Field(default=True, description="Notify when this band plays a show")
+    created_at: datetime = Field(default_factory=datetime.utcnow)
+    
+    user: "User" = Relationship()
+    vertical: "Vertical" = Relationship()
 
 class Profile(SQLModel, table=True):
     """A user's identity within a specific context or global"""