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/20260331100010_create_article_templates.up.sql

@@ -0,0 +1,20 @@
+CREATE TABLE article_templates (
+    id                          BIGSERIAL PRIMARY KEY,
+    scope                       VARCHAR(20) NOT NULL,
+    tenant_id                   BIGINT REFERENCES tenants(id),
+    origin_type                 VARCHAR(20) NOT NULL DEFAULT 'platform',
+    share_code_id               BIGINT,
+    template_key                VARCHAR(100) NOT NULL,
+    template_name               VARCHAR(200) NOT NULL,
+    schema_json                 JSONB NOT NULL DEFAULT '{}',
+    prompt_template             TEXT,
+    prompt_visibility           VARCHAR(20) NOT NULL DEFAULT 'visible',
+    protected_prompt_asset_key  TEXT,
+    card_config_json            JSONB NOT NULL DEFAULT '{}',
+    status                      VARCHAR(20) NOT NULL DEFAULT 'active',
+    version_no                  INT NOT NULL DEFAULT 1,
+    created_at                  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at                  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    deleted_at                  TIMESTAMPTZ
+);
+CREATE UNIQUE INDEX uk_template_scope_key_version_active ON article_templates(scope, tenant_id, template_key, version_no) WHERE deleted_at IS NULL;