feat add tenant users

backend/database/migrations/20251216011456_tenant_users.sql

@@ -0,0 +1,20 @@
+-- +goose Up
+-- +goose StatementBegin
+CREATE TABLE IF NOT EXISTS tenant_users(
+    id bigserial PRIMARY KEY,
+    tenant_id bigint NOT NULL,
+    user_id bigint NOT NULL,
+    role TEXT[] NOT NULL DEFAULT ARRAY['member'],
+    balance numeric(20, 8) NOT NULL DEFAULT 0,
+    status varchar(50) NOT NULL DEFAULT 'active',
+    created_at timestamptz NOT NULL DEFAULT NOW(),
+    updated_at timestamptz NOT NULL DEFAULT NOW(),
+    UNIQUE (tenant_id, user_id)
+);
+
+-- +goose StatementEnd
+-- +goose Down
+-- +goose StatementBegin
+DROP TABLE IF EXISTS tenant_users;
+
+-- +goose StatementEnd