[feat] setup master from fork with fastapi + otel

docs/003-observability-via-interfaces.md

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+# 003 Observability Via Interfaces
+
+Context
+- Logging, metrics, and tracing are required, but inner layers must stay independent from OpenTelemetry.
+- The project needs request logging and metrics middleware.
+
+Decision
+- Define observability interfaces in `usecase/interface.py`.
+- Implement those interfaces with OpenTelemetry adapters in `adapter/otel/`.
+- Use request logging middleware and metrics middleware in the FastAPI adapter.
+- Do not add a custom trace middleware by default.
+- Use standard ASGI/FastAPI OpenTelemetry instrumentation for request spans.
+
+Consequences
+- Usecases stay portable and framework-agnostic.
+- Trace behavior remains consistent with OpenTelemetry defaults.
+- Extra span enrichment can be added later without changing inner-layer contracts.