docs: add architecture documentation and slides

docs/adr/001-use-python-grpc.md

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+# ADR 001: Use Python gRPC for Auth
+
+## Status
+
+Accepted.
+
+## Context
+
+The assessment requires Python with gRPC and focuses on microservice communication.
+
+## Decision
+
+Implement the authentication boundary as an async `grpc.aio` service with protobuf contracts.
+
+## Consequences
+
+Service-to-service calls are strongly typed and efficient. Browser interaction needs a separate demo client service because native browser gRPC is not practical without grpc-web or a proxy.
+