Add Space metadata model and provisional Forgejo content ADR

Introduce tenant-scoped Space/SpaceMembership with list and detail views.
Document markdown-in-Forgejo as content SoR (ADR-0002). Run migrations on
container start so app.coulomb.social picks up the new tables.

coulomb_social/apps/spaces/urls.py

@@ -0,0 +1,9 @@
+from django.urls import path
+
+from . import views
+
+app_name = "spaces"
+
+urlpatterns = [
+    path("<slug:slug>/", views.space_detail, name="detail"),
+]