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")