chore: add OpenTelemetry observability instrumentation

[ckrough]

Summary

• Add OpenTelemetry tracing to key operations in the RAG pipeline
• Create src/infrastructure/observability/ module with setup, tracing utilities, and structlog processor
• Instrument LLM, embeddings, vectordb, cache, and RAG service operations
• Add configuration via OTEL_ENABLED, OTEL_EXPORTER, OTEL_ENDPOINT env vars

Test plan

• 14 unit tests for observability module pass
• ruff format/check passes
• bandit security scan passes
• mypy passes (pre-existing stub issues only)
• Manual verification with console exporter: OTEL_EXPORTER=console uvicorn src.main:app

Closes retriever-cd6