feat: JWT auth, /api prefix, SPA serving, OpenAPI, lean main.rs

- auth: register/login/refresh/logout w/ JWT+Argon2, protected mutations
- domain: User, RefreshSession, auth ports, Unauthorized/Forbidden errors
- presentation: context/state/factory/errors/extractors/openapi modules
- routes behind /api, SPA served from root w/ fallback
- OpenAPI Scalar at /docs
- frontend ssr:false, single-binary Dockerfile

crates/adapters/sqlite/migrations/002_users.sql

@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS users (
+    id TEXT PRIMARY KEY NOT NULL,
+    email TEXT UNIQUE NOT NULL,
+    username TEXT UNIQUE NOT NULL,
+    password_hash TEXT NOT NULL,
+    created_at TEXT NOT NULL
+);