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/views.py

@@ -0,0 +1,34 @@
+from django.contrib.auth.decorators import login_required
+from django.http import HttpRequest, HttpResponse, HttpResponseForbidden, HttpResponseNotFound
+from django.shortcuts import render
+
+from coulomb_social.apps.core.principal import build_principal
+
+from .services import get_space_for_member, spaces_for_member
+
+
+@login_required
+def space_detail(request: HttpRequest, slug: str) -> HttpResponse:
+    principal = build_principal(request, authz_resource_id="space_detail")
+    if not principal["authz_allow"]:
+        return HttpResponseForbidden(
+            f"Not authorized to view spaces ({principal['authz_reason']})."
+        )
+    member = principal.get("member")
+    space = get_space_for_member(member, slug)
+    if space is None:
+        return HttpResponseNotFound("Space not found.")
+    return render(
+        request,
+        "spaces/detail.html",
+        {
+            "principal": principal,
+            "display_name": principal["display_name"],
+            "space": space,
+        },
+    )
+
+
+# used by core.app_home — re-export list helper
+def list_spaces_context(member) -> dict:
+    return {"spaces": list(spaces_for_member(member))}