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/20260331100012_create_article_versions.up.sql

@@ -0,0 +1,12 @@
+CREATE TABLE article_versions (
+    id               BIGSERIAL PRIMARY KEY,
+    article_id       BIGINT NOT NULL REFERENCES articles(id),
+    version_no       INT NOT NULL,
+    title            VARCHAR(500),
+    html_content     TEXT,
+    markdown_content TEXT,
+    word_count       INT NOT NULL DEFAULT 0,
+    source_label     VARCHAR(100),
+    created_at       TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    CONSTRAINT uk_article_version_no UNIQUE (article_id, version_no)
+);