feat: add tenant and user management with migrations, handlers, and tests

- Implemented tenant and user management features including:
  - Tenant creation and management with associated migrations.
  - User creation and management with associated migrations.
  - Tenant membership management with associated migrations.
  - Platform user roles management with associated migrations.
  - Quota management with associated migrations.
  - Article and template management with associated migrations.
- Added HTTP handlers for templates and workspaces.
- Created tests for protected and public routes.
- Introduced a script to check tenant scope in SQL queries.
- Documented task plan for backend completion and frontend foundation.

server/migrations/20260331100005_create_tenant_plan_subscriptions.up.sql

@@ -0,0 +1,12 @@
+CREATE TABLE tenant_plan_subscriptions (
+    id         BIGSERIAL PRIMARY KEY,
+    tenant_id  BIGINT NOT NULL REFERENCES tenants(id),
+    plan_id    BIGINT NOT NULL REFERENCES plans(id),
+    start_at   TIMESTAMPTZ NOT NULL,
+    end_at     TIMESTAMPTZ NOT NULL,
+    status     VARCHAR(20) NOT NULL DEFAULT 'active',
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    deleted_at TIMESTAMPTZ
+);
+CREATE UNIQUE INDEX idx_tenant_plan_active ON tenant_plan_subscriptions(tenant_id) WHERE status = 'active' AND deleted_at IS NULL;