Render space pages from Forgejo markdown (CSOC-WP-0004-T04)

Fetch raw files from Forgejo, sanitize markdown to HTML, and show them on
space detail. Ship a public demo fixture path and seed_demo_space command.

docs/space-fixtures/demo/pages/index.md

@@ -0,0 +1,23 @@
+# Demo space
+
+Welcome to the **coulomb.social** rebuild on Railiance.
+
+This page is markdown stored in git (Forgejo) and rendered by the app
+(`CSOC-WP-0004-T04`).
+
+## What you are seeing
+
+| Layer | Source |
+|-------|--------|
+| Membership / shell | NetKingdom OIDC + app session |
+| Space metadata | Postgres `spaces_space` |
+| Page body | This file in `coulomb/coulomb-social` |
+
+## Next
+
+- Bind real product spaces to dedicated Forgejo repos (ADR-0002).
+- Edit content in git; refresh the app to re-fetch (short cache TTL).
+
+```python
+print("agents can edit this content in git")
+```