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/20260331100021_create_brand_keywords.up.sql

@@ -0,0 +1,11 @@
+CREATE TABLE brand_keywords (
+    id         BIGSERIAL PRIMARY KEY,
+    tenant_id  BIGINT NOT NULL REFERENCES tenants(id),
+    brand_id   BIGINT NOT NULL REFERENCES brands(id),
+    name       VARCHAR(200) 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 uk_brand_keyword_name_active ON brand_keywords(brand_id, name) WHERE deleted_at IS NULL;