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

[constk]

Problem

Distributed tracing has to be wired before the first feature ships, or instrumentation never gets retrofitted.

Proposed solution

Port src/observability/{logging.py, spans.py, tracing.py} from Teller. Tracing: OTel SDK + OTLP gRPC exporter pointed at OTEL_EXPORTER_OTLP_ENDPOINT (Jaeger via docker-compose). Logging: structured JSON logs correlated with active span ID. Span helpers: official OTel semantic-convention attribute names only (per feedback_otel_semconv memory). Wire FastAPI auto-instrumentation in src/api/main.py.

Acceptance criteria

• Hitting /api/v1/health produces a span visible in Jaeger UI.
• Logs are JSON, single-line, with trace_id and span_id fields.
• tests/test_observability.py asserts span creation and semconv attribute names.
• No custom-named OTel attributes; only semconv-defined ones.

Priority rationale

High: observability retrofitted is observability never done. Locks in correct tracing posture from day one.

Depends on

#17

[constk]

Merged in PR #56.