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

alembic/versions/0124_unified_task_system.py

@@ -20,7 +20,27 @@ branch_labels = None
 depends_on = None
 
 
+def _table_exists(conn, table_name: str) -> bool:
+    """True when the table exists (dual-path convergence, Gate B).
+
+    On a fresh install the tasks plugin SQL migration has not run yet when
+    Alembic reaches this revision — skip instead of failing. The plugin-side
+    convergence migration adds the same columns/indexes. The legacy-data
+    backfills below only matter for pre-existing rows and are correctly
+    empty on a fresh install.
+    """
+    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
+
     # ── Add new columns to tasks ────────────────────────────────────────────
     op.add_column("tasks", sa.Column("assignee_type", sa.String(20), nullable=False, server_default="user"))
     op.add_column("tasks", sa.Column("assignee_id", PGUUID(as_uuid=True), nullable=True))