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/main.py

@@ -1,14 +1,31 @@
 """FastAPI application entrypoint.
 
-Thin by design: wire settings and routers, expose the OpenAPI contract. All
-domain logic lives in the service layer (added with the data model). The
-versioned API will mount under ``/api/v1``; health probes stay at the root.
+Thin by design: wire settings, routers, and error handling, and expose the
+OpenAPI contract. All domain logic lives in the service layer; the privacy
+engine is the single enforcement point for reads.
 """
 
-from fastapi import FastAPI
+from fastapi import FastAPI, Request
+from fastapi.responses import JSONResponse
 
 from app.api.health import router as health_router
+from app.api.v1 import api_router
 from app.core.config import get_settings
+from app.services.exceptions import Conflict, Forbidden, NotFound
+
+
+def _register_error_handlers(app: FastAPI) -> None:
+    @app.exception_handler(NotFound)
+    async def _not_found(request: Request, exc: NotFound) -> JSONResponse:
+        return JSONResponse(status_code=404, content={"detail": str(exc) or "not found"})
+
+    @app.exception_handler(Forbidden)
+    async def _forbidden(request: Request, exc: Forbidden) -> JSONResponse:
+        return JSONResponse(status_code=403, content={"detail": str(exc) or "forbidden"})
+
+    @app.exception_handler(Conflict)
+    async def _conflict(request: Request, exc: Conflict) -> JSONResponse:
+        return JSONResponse(status_code=409, content={"detail": str(exc) or "conflict"})
 
 
 def create_app() -> FastAPI:
@@ -19,6 +36,8 @@ def create_app() -> FastAPI:
         description="Provenance API — family and land provenance.",
     )
     app.include_router(health_router)
+    app.include_router(api_router)
+    _register_error_handlers(app)
     return app