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/audit.py

@@ -0,0 +1,37 @@
+"""Audit logging. Every mutation records an append-only AuditEntry attributing
+the change to a User (or the assistant principal acting for a User). Staged on
+the session; the caller commits as part of its unit of work.
+"""
+
+import uuid
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.audit import AuditEntry
+from app.models.enums import AuditActorType
+
+
+def record_audit(
+    session: AsyncSession,
+    *,
+    action: str,
+    entity_type: str,
+    entity_id: uuid.UUID | None = None,
+    tree_id: uuid.UUID | None = None,
+    actor_user_id: uuid.UUID | None = None,
+    actor_type: AuditActorType = AuditActorType.user,
+    before: dict | None = None,
+    after: dict | None = None,
+) -> AuditEntry:
+    entry = AuditEntry(
+        action=action,
+        entity_type=entity_type,
+        entity_id=entity_id,
+        tree_id=tree_id,
+        actor_user_id=actor_user_id,
+        actor_type=actor_type,
+        before=before,
+        after=after,
+    )
+    session.add(entry)
+    return entry