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.
