Add unit/integration tests across backend handlers, auth, caddy and
util packages, plus frontend component and API tests with the new
test setup. Update CI workflow and Vite config to run the suites.
Introduce per-host plugin support: Caddy module discovery, module
config endpoints, plugin schemas (basicauth, cloudflare) and the
host plugins API. On the frontend, add the Plugins page, plugin/host
plugin dialogs and schema-driven form fields.
Support CPM_AUTH_MODE=local|oidc. OIDC uses go-oidc with JIT user
provisioning and an optional email-domain allowlist. Adds auth config,
login and callback endpoints, a JWT signing-key bootstrap and
Provider/Subject fields on the user model. Includes OIDC allowlist tests.
