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/schemas/tree.py

@@ -0,0 +1,23 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict
+
+from app.models.enums import TreeVisibility
+
+
+class TreeCreate(BaseModel):
+    name: str
+    description: str | None = None
+    visibility: TreeVisibility = TreeVisibility.private
+
+
+class TreeRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    name: str
+    description: str | None
+    visibility: TreeVisibility
+    owner_id: uuid.UUID
+    created_at: datetime