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/models.py

@@ -0,0 +1,115 @@
+"""Space metadata — content bodies live in Forgejo (markdown), not here."""
+
+from __future__ import annotations
+
+import re
+import uuid
+
+from django.core.exceptions import ValidationError
+from django.db import models
+from django.utils.text import slugify
+
+
+_SLUG_RE = re.compile(r"^[a-z0-9]+(?:-[a-z0-9]+)*$")
+
+
+def validate_space_slug(value: str) -> None:
+    if not _SLUG_RE.match(value or ""):
+        raise ValidationError(
+            "Slug must be lowercase alphanumeric with single hyphens (e.g. my-space)."
+        )
+
+
+class Space(models.Model):
+    """Tenant-scoped co-creation space.
+
+    Postgres holds metadata and Forgejo binding pointers only. Long-form pages
+    are markdown in the bound repository (ADR-0002).
+    """
+
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    tenant_id = models.CharField(max_length=64, db_index=True)
+    slug = models.SlugField(max_length=80, validators=[validate_space_slug])
+    title = models.CharField(max_length=255)
+    description = models.TextField(blank=True)
+    created_by = models.ForeignKey(
+        "members.Member",
+        on_delete=models.PROTECT,
+        related_name="spaces_created",
+        null=True,
+        blank=True,
+    )
+    # Optional content binding (Forgejo) — empty until T04 wires the read path
+    forgejo_owner = models.CharField(
+        max_length=128,
+        blank=True,
+        help_text="Forgejo org or user owning the repo (e.g. coulomb).",
+    )
+    forgejo_repo = models.CharField(
+        max_length=128,
+        blank=True,
+        help_text="Repository name (e.g. space-my-space).",
+    )
+    default_branch = models.CharField(max_length=128, blank=True, default="main")
+    content_root = models.CharField(
+        max_length=255,
+        blank=True,
+        default="pages",
+        help_text="Path prefix inside the repo for markdown pages.",
+    )
+    is_active = models.BooleanField(default=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(
+                fields=["tenant_id", "slug"],
+                name="spaces_space_tenant_slug_uniq",
+            ),
+        ]
+        indexes = [
+            models.Index(fields=["tenant_id", "is_active"]),
+        ]
+        ordering = ["title", "slug"]
+
+    def __str__(self) -> str:
+        return f"{self.title} ({self.slug})@{self.tenant_id}"
+
+    @property
+    def has_content_binding(self) -> bool:
+        return bool(self.forgejo_owner and self.forgejo_repo)
+
+    @classmethod
+    def suggest_slug(cls, title: str) -> str:
+        return slugify(title)[:80] or "space"
+
+
+class SpaceMembership(models.Model):
+    """Optional explicit membership; v1 list also allows tenant-wide visibility."""
+
+    class Role(models.TextChoices):
+        OWNER = "owner", "Owner"
+        MEMBER = "member", "Member"
+        VIEWER = "viewer", "Viewer"
+
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    space = models.ForeignKey(Space, on_delete=models.CASCADE, related_name="memberships")
+    member = models.ForeignKey(
+        "members.Member",
+        on_delete=models.CASCADE,
+        related_name="space_memberships",
+    )
+    role = models.CharField(max_length=16, choices=Role.choices, default=Role.MEMBER)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(
+                fields=["space", "member"],
+                name="spaces_membership_space_member_uniq",
+            ),
+        ]
+
+    def __str__(self) -> str:
+        return f"{self.member_id}@{self.space.slug}:{self.role}"