feat: add datatable_migrations table

backend/migrations/20260617081932_datatable_migrations.down.sql

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+DROP TABLE datatable_migrations;

backend/migrations/20260617081932_datatable_migrations.up.sql

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+-- SQL migrations defined per workspace for data tables.
+-- `timestamp` is the migration version (YYYYMMDDHHMMSS), recorded as `version`
+-- in the data table's `_wm_migrations` table once applied.
+CREATE TABLE datatable_migrations (
+    workspace_id VARCHAR(50) NOT NULL REFERENCES workspace(id) ON DELETE CASCADE,
+    timestamp BIGINT NOT NULL,
+    name VARCHAR(255) NOT NULL,
+    code_up TEXT NOT NULL,
+    code_down TEXT,
+    PRIMARY KEY (workspace_id, timestamp)
+);
+
+-- No standalone index on workspace_id: the (workspace_id, timestamp) primary-key
+-- btree already serves `WHERE workspace_id = $1` lookups via its leading column.
+
+GRANT ALL ON datatable_migrations TO windmill_user;
+GRANT ALL ON datatable_migrations TO windmill_admin;