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/exceptions.py

@@ -0,0 +1,18 @@
+"""Domain errors. The API layer maps these to HTTP status codes so services
+stay transport-agnostic."""
+
+
+class DomainError(Exception):
+    """Base for domain-level errors."""
+
+
+class NotFound(DomainError):
+    """Requested entity does not exist (or is soft-deleted / not visible)."""
+
+
+class Forbidden(DomainError):
+    """Caller lacks the required role for this action."""
+
+
+class Conflict(DomainError):
+    """Operation conflicts with current state (e.g. duplicate email)."""