Add layered service/API for tenancy and people with the privacy seam

Wires the data model through repository -> service -> API/v1. The privacy engine (app/services/privacy.py) is the single enforcement point: every read resolves visibility there (tree role, tree visibility, per-person override; living-person redaction is a marked Phase 2 TODO). All writes record an attributable AuditEntry.

Endpoints: POST /users (open dev bootstrap until auth), GET /users/me, POST/GET /trees, GET /trees/{id}, and POST/GET /trees/{id}/persons. Authn is a temporary X-User-Id header shim; authz is membership-based (owner/editor/viewer). Domain errors map to 401/403/404/409. Verified on the deploy target: private tree -> 403 for non-members, missing actor -> 401, audit log populated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/services/privacy.py

@@ -0,0 +1,62 @@
+"""The privacy engine — the single enforcement point for visibility.
+
+INVARIANT (CLAUDE.md #2): every read path resolves visibility here. Do not add a
+query path that returns rows to a caller without first passing through this
+module. Effective visibility is a function of the viewer's role on the tree, the
+tree's visibility, the per-person override, and (Phase 2) living-person status.
+"""
+
+import enum
+import uuid
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.enums import MembershipRole, PersonPrivacy, TreeVisibility
+from app.models.person import Person
+from app.models.tree import Tree, TreeMembership
+
+
+class Visibility(enum.StrEnum):
+    full = "full"
+    redacted = "redacted"
+    hidden = "hidden"
+
+
+async def get_membership_role(
+    session: AsyncSession, user_id: uuid.UUID | None, tree_id: uuid.UUID
+) -> MembershipRole | None:
+    if user_id is None:
+        return None
+    stmt = select(TreeMembership.role).where(
+        TreeMembership.tree_id == tree_id,
+        TreeMembership.user_id == user_id,
+    )
+    return (await session.execute(stmt)).scalar_one_or_none()
+
+
+async def can_view_tree(session: AsyncSession, *, user_id: uuid.UUID | None, tree: Tree) -> bool:
+    if tree.deleted_at is not None:
+        return False
+    if await get_membership_role(session, user_id, tree.id) is not None:
+        return True
+    return tree.visibility in (TreeVisibility.public, TreeVisibility.unlisted)
+
+
+async def can_edit_tree(session: AsyncSession, *, user_id: uuid.UUID | None, tree: Tree) -> bool:
+    role = await get_membership_role(session, user_id, tree.id)
+    return role in (MembershipRole.owner, MembershipRole.editor)
+
+
+async def person_visibility(
+    session: AsyncSession, *, user_id: uuid.UUID | None, tree: Tree, person: Person
+) -> Visibility:
+    if not await can_view_tree(session, user_id=user_id, tree=tree):
+        return Visibility.hidden
+    if await get_membership_role(session, user_id, tree.id) is not None:
+        return Visibility.full
+    # Non-member viewing a public/unlisted tree:
+    if person.privacy == PersonPrivacy.private:
+        return Visibility.hidden
+    # TODO(Phase 2): redact living people for non-members (ARCHITECTURE §6).
+    return Visibility.full