Implement flexible entity relation system (Phases A–E)

Declarative relation registry via defrelation s-expressions with
cardinality enforcement (one-to-one, one-to-many, many-to-many),
registry-aware relate/unrelate/can-relate API endpoints, generic
container-nav fragment, and relation-driven UI components.

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

relations/alembic/versions/0002_add_relation_type_and_metadata.py

@@ -0,0 +1,52 @@
+"""Add relation_type and metadata columns to container_relations
+
+Revision ID: relations_0002
+Revises: relations_0001
+Create Date: 2026-02-28
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "relations_0002"
+down_revision = "relations_0001"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "container_relations",
+        sa.Column("relation_type", sa.String(64), nullable=True),
+    )
+    op.add_column(
+        "container_relations",
+        sa.Column("metadata", sa.JSON(), nullable=True),
+    )
+    op.create_index(
+        "ix_container_relations_relation_type",
+        "container_relations",
+        ["relation_type", "parent_type", "parent_id"],
+    )
+
+    # Backfill relation_type for existing rows based on parent/child type pairs
+    op.execute("""
+        UPDATE container_relations
+        SET relation_type = CASE
+            WHEN parent_type = 'page' AND child_type = 'market'
+                THEN 'page->market'
+            WHEN parent_type = 'page' AND child_type = 'calendar'
+                THEN 'page->calendar'
+            WHEN parent_type = 'page' AND child_type = 'menu_node'
+                THEN 'page->menu_node'
+        END
+        WHERE relation_type IS NULL
+          AND parent_type = 'page'
+          AND child_type IN ('market', 'calendar', 'menu_node')
+    """)
+
+
+def downgrade() -> None:
+    op.drop_index("ix_container_relations_relation_type", "container_relations")
+    op.drop_column("container_relations", "metadata")
+    op.drop_column("container_relations", "relation_type")