Epic: Guarded Observability Stack

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Implement a comprehensive OpenTelemetry-based observability solution (traces, metrics, logs) with guard-rails to avoid performance, cost, and security pitfalls.\n\nAcceptance criteria:\n1. Child issues completed and referenced.\n2. Observability can be enabled/disabled and endpoints configured via AppSettings.\n3. CI & tests pass with OBS_ENABLED both on and off.\n4. Documentation updated.\n\nContext: Based on the observability plan discussed in https://github.com/pmcfadin/mcp-observability/blob/main/docs/observability/python_fastapi.md.

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