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/admin.py

@@ -0,0 +1,34 @@
+from django.contrib import admin
+
+from .models import Space, SpaceMembership
+
+
+class SpaceMembershipInline(admin.TabularInline):
+    model = SpaceMembership
+    extra = 0
+    raw_id_fields = ("member",)
+
+
+@admin.register(Space)
+class SpaceAdmin(admin.ModelAdmin):
+    list_display = (
+        "title",
+        "slug",
+        "tenant_id",
+        "is_active",
+        "forgejo_owner",
+        "forgejo_repo",
+        "updated_at",
+    )
+    list_filter = ("tenant_id", "is_active")
+    search_fields = ("title", "slug", "forgejo_repo")
+    prepopulated_fields = {"slug": ("title",)}
+    raw_id_fields = ("created_by",)
+    inlines = [SpaceMembershipInline]
+
+
+@admin.register(SpaceMembership)
+class SpaceMembershipAdmin(admin.ModelAdmin):
+    list_display = ("space", "member", "role", "created_at")
+    list_filter = ("role",)
+    raw_id_fields = ("space", "member")