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.

tests/test_spaces.py

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+import pytest
+from django.urls import reverse
+
+from coulomb_social.apps.members.models import Member, User
+from coulomb_social.apps.spaces.models import Space, SpaceMembership
+from coulomb_social.apps.spaces.services import spaces_for_member
+
+
+def _login(client, settings, *, subject: str, tenant: str, name: str):
+    settings.DEBUG = True
+    settings.OIDC_ENABLED = False
+    settings.DEFAULT_TENANT_ID = tenant
+    return client.post(
+        reverse("identity:dev_login"),
+        {
+            "subject": subject,
+            "issuer": "https://local.dev/issuer",
+            "name": name,
+            "tenant": tenant,
+        },
+    )
+
+
+@pytest.mark.django_db
+def test_spaces_list_empty_for_tenant(client, settings):
+    _login(client, settings, subject="u1", tenant="tenant:a", name="User One")
+    r = client.get(reverse("core:app_home"))
+    assert r.status_code == 200
+    assert b"No spaces yet" in r.content
+
+
+@pytest.mark.django_db
+def test_spaces_list_shows_same_tenant_only(client, settings):
+    _login(client, settings, subject="u1", tenant="tenant:a", name="User One")
+    member = Member.objects.get(subject="u1")
+    Space.objects.create(
+        tenant_id="tenant:a",
+        slug="alpha",
+        title="Alpha Space",
+        created_by=member,
+    )
+    Space.objects.create(
+        tenant_id="tenant:b",
+        slug="beta",
+        title="Beta Space",
+    )
+
+    r = client.get(reverse("core:app_home"))
+    assert r.status_code == 200
+    assert b"Alpha Space" in r.content
+    assert b"Beta Space" not in r.content
+    assert b"No spaces yet" not in r.content
+
+
+@pytest.mark.django_db
+def test_space_detail_tenant_isolation(client, settings):
+    _login(client, settings, subject="u1", tenant="tenant:a", name="User One")
+    Space.objects.create(tenant_id="tenant:b", slug="secret", title="Secret")
+    r = client.get(reverse("spaces:detail", kwargs={"slug": "secret"}))
+    assert r.status_code == 404
+
+
+@pytest.mark.django_db
+def test_space_detail_ok(client, settings):
+    _login(client, settings, subject="u1", tenant="tenant:a", name="User One")
+    member = Member.objects.get(subject="u1")
+    Space.objects.create(
+        tenant_id="tenant:a",
+        slug="lab",
+        title="Lab",
+        description="Research lab",
+        created_by=member,
+        forgejo_owner="coulomb",
+        forgejo_repo="space-lab",
+    )
+    r = client.get(reverse("spaces:detail", kwargs={"slug": "lab"}))
+    assert r.status_code == 200
+    assert b"Lab" in r.content
+    assert b"coulomb/space-lab" in r.content
+
+
+@pytest.mark.django_db
+def test_membership_narrows_visibility(client, settings):
+    _login(client, settings, subject="u1", tenant="tenant:a", name="User One")
+    member = Member.objects.get(subject="u1")
+    other_user = User.objects.create_user(username="other")
+    other = Member.objects.create(
+        tenant_id="tenant:a",
+        user=other_user,
+        user_engine_user_id="usr_other",
+        issuer="https://local.dev/issuer",
+        subject="other-sub",
+        display_name="Other",
+    )
+    open_space = Space.objects.create(
+        tenant_id="tenant:a", slug="open", title="Open", created_by=other
+    )
+    closed = Space.objects.create(
+        tenant_id="tenant:a", slug="closed", title="Closed", created_by=other
+    )
+    # Any membership in tenant switches mode to membership-scoped
+    SpaceMembership.objects.create(
+        space=closed, member=other, role=SpaceMembership.Role.OWNER
+    )
+    SpaceMembership.objects.create(
+        space=open_space, member=member, role=SpaceMembership.Role.MEMBER
+    )
+
+    visible = list(spaces_for_member(member).values_list("slug", flat=True))
+    assert "open" in visible
+    assert "closed" not in visible