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/services.py

@@ -0,0 +1,39 @@
+"""Space queries scoped by tenant membership."""
+
+from __future__ import annotations
+
+from django.db.models import Q, QuerySet
+
+from coulomb_social.apps.members.models import Member
+
+from .models import Space, SpaceMembership
+
+
+def spaces_for_member(member: Member | None) -> QuerySet[Space]:
+    """Spaces visible to this member (same tenant, active).
+
+    Explicit SpaceMembership grants access; if none exist for the tenant yet,
+    all active tenant spaces are listed (operator-seeded MVP). Once any
+    memberships exist for the tenant, only membership + created_by apply.
+    """
+    if member is None:
+        return Space.objects.none()
+
+    qs = Space.objects.filter(tenant_id=member.tenant_id, is_active=True)
+    tenant_has_memberships = SpaceMembership.objects.filter(
+        space__tenant_id=member.tenant_id
+    ).exists()
+    if not tenant_has_memberships:
+        return qs.order_by("title", "slug")
+
+    return (
+        qs.filter(Q(memberships__member=member) | Q(created_by=member))
+        .distinct()
+        .order_by("title", "slug")
+    )
+
+
+def get_space_for_member(member: Member | None, slug: str) -> Space | None:
+    if member is None:
+        return None
+    return spaces_for_member(member).filter(slug=slug).first()