inertia: server-side rendering (or server-rendered <head>) for public pages #535

Context: second migration-gap sweep for porting a large production FastAPI/Celery GIS app (gis-data-portal: MinIO-hosted PMTiles/COG maps embedded cross-origin, multi-worker web + prefork Celery, TS web client + Flutter mobile client) onto SimpleModule. Verified against framework main @ ec46dc4b and smpy_modules main @ 1284466. Portal paths are given for reference.

Problem

framework/inertia/simple_module_inertia/inertia.py:136 hard-codes is_ssr=False and ignores the SSR config (config.py:25). templating.py:72 emits an empty root. As a result:

• public pages (landing, public catalog, CMS pages) are blank to crawlers and link-unfurlers that don't run JS;
• pagebuilder's SEO and OG tags are set with React <Head> (smpy_modules/modules/pagebuilder/pagebuilder/pages/PublicPage.tsx:92), so they only exist after hydration. Slack, LinkedIn and many crawlers never see them.

The portal is SSR today (TanStack Start: frontend/vite.config.ts:35, head at src/routes/__root.tsx:126). Moving would regress SEO and share previews.

Proposal

Two steps, so the cheap part can ship first:

1. Server-rendered head: a head prop or contract that Inertia views fill (title, meta, OG, canonical, JSON-LD), rendered into index.html by the template on the first response. pagebuilder and news use it.
2. Optional Inertia SSR: honour the config. Run the Inertia SSR server (Node sidecar) when enabled, fall back to CSR if it is unreachable, and document the deploy shape.

Acceptance

curl on a public pagebuilder page returns the correct <title>, og:* and canonical tags without running JS. With SSR enabled, the page body HTML is also present.