feat: add OpenTelemetry error reporting

[matheus1lva]

Summary

Report Worker errors as OpenTelemetry log records over OTLP/HTTP. Cloudflare Workers can't run the OpenTelemetry Node SDK (its OTLP exporter is Node-coupled), so the OTLP/HTTP log payload is built and sent directly via fetch — still standard OTLP. Vendor-neutral: point OTEL_EXPORTER_OTLP_ENDPOINT (and OTEL_EXPORTER_OTLP_HEADERS for auth) at any OTLP backend. No-op until configured.

How to review

• src/observability.ts: captureError(ctx, env, error) POSTs an OTLP LogsData payload (ERROR severity, exception.* attributes) to ${OTEL_EXPORTER_OTLP_ENDPOINT}/v1/logs. Auth headers come from OTEL_EXPORTER_OTLP_HEADERS. Uses ctx.waitUntil so the export completes after the response is returned — no added request latency.
• src/index.ts: the unexpected-error branch (non-ApiError, 500) calls captureError; ApiError (expected 4xx) paths are unchanged. OTEL_* fields added to the Env type.

Test plan

• Manual: set OTEL_EXPORTER_OTLP_ENDPOINT (+ OTEL_EXPORTER_OTLP_HEADERS), trigger the 500 branch, confirm the log record reaches the backend.
• Manual: leave the endpoint unset, confirm nothing is exported and responses are unchanged.
• Automated: bun run test (33 tests) and bun run typecheck.

Risk / impact

Low. With the OTLP endpoint unset (the default), reporting is a no-op and behavior is unchanged. No funds, auth, or migration paths touched.