feat(boards): add max_agents field to board models and enforce limits

backend/migrations/versions/4c1f5e2a7b9d_add_boards_max_agents.py

@@ -0,0 +1,37 @@
+"""Add max_agents field to boards.
+
+Revision ID: 4c1f5e2a7b9d
+Revises: c9d7e9b6a4f2
+Create Date: 2026-02-14 00:00:00.000000
+
+"""
+
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "4c1f5e2a7b9d"
+down_revision = "c9d7e9b6a4f2"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    """Add required boards.max_agents column with a safe backfill default."""
+    op.add_column(
+        "boards",
+        sa.Column(
+            "max_agents",
+            sa.Integer(),
+            nullable=False,
+            server_default=sa.text("1"),
+        ),
+    )
+    op.alter_column("boards", "max_agents", server_default=None)
+
+
+def downgrade() -> None:
+    """Remove boards.max_agents column."""
+    op.drop_column("boards", "max_agents")