docs: add architecture documentation and slides

docs/adr/005-fastapi-demo-client.md

@@ -0,0 +1,18 @@
+# ADR 005: Add FastAPI Demo Client Service
+
+## Status
+
+Accepted.
+
+## Context
+
+The gRPC service needs a simple browser demo for interviews and reviewers.
+
+## Decision
+
+Add `gapido_demo`, a small FastAPI backend-for-frontend that serves static UI and calls the auth service over gRPC.
+
+## Consequences
+
+The demo shows how a separate microservice consumes auth. It remains separate from the auth service package and can be exposed safely through Caddy.
+