feat: observability setup (OTel SDK, OTLP exporter, structured logging, span helpers) #36

Job	Run time
Lint & Format	12s
Pre-commit	1m 10s
Architecture (import-linter)	12s
Coverage	23s
Branch-protection contexts sync	10s
Type Check	19s
Unit tests	20s
Commit-type sync	11s
	2m 57s

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