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/20260331100003_create_platform_user_roles.up.sql

@@ -0,0 +1,9 @@
+CREATE TABLE platform_user_roles (
+    id            BIGSERIAL PRIMARY KEY,
+    user_id       BIGINT NOT NULL REFERENCES users(id),
+    platform_role VARCHAR(20) NOT NULL,
+    created_at    TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at    TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    deleted_at    TIMESTAMPTZ
+);
+CREATE UNIQUE INDEX uk_platform_user_role_active ON platform_user_roles(user_id, platform_role) WHERE deleted_at IS NULL;