fix(gate-b): fresh-db install path - conditional guards on plugin-table migrations + dual-path convergence migrations

alembic/versions/0127_drop_tasks_contact_id_fk.py

@@ -12,6 +12,7 @@ Revises: 0126
 """
 
 from alembic import op
+import sqlalchemy as sa
 
 revision = "0127"
 down_revision = "0126"
@@ -19,7 +20,19 @@ branch_labels = None
 depends_on = None
 
 
+def _table_exists(conn, table_name: str) -> bool:
+    """True when the table exists (dual-path convergence, Gate B)."""
+    row = conn.execute(
+        sa.text("SELECT to_regclass(:tname) IS NOT NULL"),
+        {"tname": f"public.{table_name}"},
+    ).scalar()
+    return bool(row)
+
+
 def upgrade() -> None:
+    conn = op.get_bind()
+    if not _table_exists(conn, "tasks"):
+        return
     # Drop the FK constraint on tasks.contact_id
     op.drop_constraint("tasks_contact_id_fkey", "tasks", type_="foreignkey")