feat: request tracing and structured logs, enforced API versions, RFC ETag matching

[Pro-builder]

Closes #1788
Closes #1789
Closes #1790
Closes #1791

#1788 ETag and conditional requests

• isETagMatch (compression.ts, used by server.ts via sendJson) now does the RFC 9110 weak comparison: comma-separated lists and W/ tags match, so CDNs that weaken the tag still get a 304.
• Integration test in server.test.ts: ETag emitted, 304 for exact, list and weak matches, 200 for a stale tag. Unit tests in compression.test.ts.
• docs/etag-caching.md now describes the real code path.

#1789 API versioning

• versionRegistry reflects what is served: v1 active and default.
• resolveVersion rejects an explicitly requested unknown version (path, header or query) with 400 VERSION_NOT_FOUND instead of serving the default. Fixes the 4 failing tests in apiVersioning.test.ts.
• createVersionMiddleware mounted in server.ts and createApiServer.ts; responses carry api-version.
• docs/api-versioning.md: current state, strategy and deprecation policy (6 month minimum window, headers, 410 on sunset).

#1790 Structured request logging

• New backend/shared/middleware/requestObservability.ts, mounted in both servers.
• X-Request-Id kept from the caller when safe, otherwise a UUID; echoed and used as the log correlationId for everything logged during the request.
• One http.request log per request with requestId, traceId, spanId, method, path, status, durationMs.
• No PII: no query strings, headers, bodies or IPs; email and long number path segments masked. Tested.
• docs/api-logging.md documents the format.

#1791 Distributed tracing

• The same middleware creates a server span that continues the incoming traceparent and exports through createTracerFromEnv (OTLP when OTEL_EXPORTER_OTLP_ENDPOINT is set).
• Logs carry traceId/spanId and spans carry request.id.
• server.ts accepts an injected tracer for tests.
• docs/distributed-tracing.md updated with the real backend setup and log correlation.

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