feat(db): add RehearsalSession table, songs.session_id, songs.tags

Introduces the rehearsal session concept: each YYMMDD folder in
Nextcloud maps to one RehearsalSession row (band_id + date unique).
Songs gain a nullable session_id FK and a tags TEXT[] column for
manual labeling (searchable by name, tag, BPM, key).

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

api/alembic/versions/0004_rehearsal_sessions.py

@@ -0,0 +1,49 @@
+"""Add rehearsal_sessions table, songs.session_id, songs.tags.
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-03-29
+"""
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import ARRAY, UUID
+
+revision = "0004"
+down_revision = "0003"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "rehearsal_sessions",
+        sa.Column("id", UUID(as_uuid=True), primary_key=True),
+        sa.Column("band_id", UUID(as_uuid=True), sa.ForeignKey("bands.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("date", sa.DateTime(timezone=False), nullable=False),
+        sa.Column("nc_folder_path", sa.Text()),
+        sa.Column("label", sa.String(255)),
+        sa.Column("notes", sa.Text()),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.UniqueConstraint("band_id", "date", name="uq_session_band_date"),
+    )
+    op.create_index("ix_rehearsal_sessions_band_id", "rehearsal_sessions", ["band_id"])
+
+    op.add_column("songs", sa.Column(
+        "session_id", UUID(as_uuid=True),
+        sa.ForeignKey("rehearsal_sessions.id", ondelete="SET NULL"),
+        nullable=True,
+    ))
+    op.create_index("ix_songs_session_id", "songs", ["session_id"])
+
+    op.add_column("songs", sa.Column(
+        "tags", ARRAY(sa.Text()), nullable=False, server_default="{}",
+    ))
+
+
+def downgrade() -> None:
+    op.drop_column("songs", "tags")
+    op.drop_index("ix_songs_session_id", table_name="songs")
+    op.drop_column("songs", "session_id")
+    op.drop_index("ix_rehearsal_sessions_band_id", table_name="rehearsal_sessions")
+    op.drop_table("rehearsal_sessions")

api/src/rehearsalhub/db/models.py

@@ -84,6 +84,9 @@ class Band(Base):
     songs: Mapped[list[Song]] = relationship(
         "Song", back_populates="band", cascade="all, delete-orphan"
     )
+    sessions: Mapped[list[RehearsalSession]] = relationship(
+        "RehearsalSession", back_populates="band", cascade="all, delete-orphan"
+    )
 
 
 class BandMember(Base):
@@ -128,6 +131,29 @@ class BandInvite(Base):
     creator: Mapped[Member] = relationship("Member", foreign_keys=[created_by])
 
 
+# ── Rehearsal Sessions ────────────────────────────────────────────────────────
+
+
+class RehearsalSession(Base):
+    __tablename__ = "rehearsal_sessions"
+    __table_args__ = (UniqueConstraint("band_id", "date", name="uq_session_band_date"),)
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    band_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("bands.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    date: Mapped[datetime] = mapped_column(DateTime(timezone=False), nullable=False)
+    nc_folder_path: Mapped[Optional[str]] = mapped_column(Text)
+    label: Mapped[Optional[str]] = mapped_column(String(255))
+    notes: Mapped[Optional[str]] = mapped_column(Text)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+
+    band: Mapped[Band] = relationship("Band", back_populates="sessions")
+    songs: Mapped[list[Song]] = relationship("Song", back_populates="session")
+
+
 # ── Songs ─────────────────────────────────────────────────────────────────────
 
 
@@ -138,9 +164,13 @@ class Song(Base):
     band_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), ForeignKey("bands.id", ondelete="CASCADE"), nullable=False, index=True
     )
+    session_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("rehearsal_sessions.id", ondelete="SET NULL"), index=True
+    )
     title: Mapped[str] = mapped_column(String(500), nullable=False)
     nc_folder_path: Mapped[Optional[str]] = mapped_column(Text)
     status: Mapped[str] = mapped_column(String(20), nullable=False, default="jam")
+    tags: Mapped[list[str]] = mapped_column(ARRAY(Text), default=list, nullable=False)
     global_key: Mapped[Optional[str]] = mapped_column(String(30))
     global_bpm: Mapped[Optional[float]] = mapped_column(Numeric(6, 2))
     notes: Mapped[Optional[str]] = mapped_column(Text)
@@ -155,6 +185,7 @@ class Song(Base):
     )
 
     band: Mapped[Band] = relationship("Band", back_populates="songs")
+    session: Mapped[Optional[RehearsalSession]] = relationship("RehearsalSession", back_populates="songs")
     creator: Mapped[Optional[Member]] = relationship("Member", back_populates="authored_songs")
     versions: Mapped[list[AudioVersion]] = relationship(
         "AudioVersion", back_populates="song", cascade="all, delete-orphan"