Add frontend error collector, CORS/ingest guards, and static doc routes (Phase 3)

- Add static/otel-rum.js: a framework-free browser script that captures
  uncaught errors, unhandled promise rejections and failed resource loads,
  translating them into OTLP log records POSTed to the existing /v1/logs
  endpoint (source: frontend attribute) so they flow through the same
  history/search/issue pipeline as backend telemetry.
- Add correlation-id support: traceHeaders() (manual) and same-origin
  fetch auto-instrumentation inject a W3C traceparent header, so a
  frontend error and the backend spans it triggered share a trace id and
  can be found together via GET /api/traces/{traceId}.
- Add CORS handling (withCORS in cmd/server/main.go) on the OTLP HTTP
  endpoints, required for any browser-origin POST.
- Add an optional shared ingest key (OTEL_INGEST_KEY, via '?key=' or
  X-Otel-Ingest-Key) and a per-IP rate limit (20 rps/burst 50) on OTLP
  HTTP ingestion, to scope/protect a public endpoint fed by a browser
  script. Verified end-to-end (CORS preflight, cross-origin POST, ingest
  key enforcement, rate limiting) against a running server.
- Serve static/otel-rum.js and agents_example.md as static HTTP routes so
  both the script and the agent instructions doc are fetchable directly
  from the base URL.
- Ignore the separately-cloned otel-ui/ frontend repo (has its own .git;
  not meant to be nested into this repo's history).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

agents_example.md

@@ -1,6 +1,6 @@
 # AI Agent Debugging with OTEL Collector
 
-This document provides instructions for AI agents on how to utilize the OTEL Collector's debugging features to search and fetch telemetry data (traces, logs, and metrics).
+This document provides instructions for AI agents on how to utilize the OTEL Collector's debugging features to search and fetch telemetry data (traces, logs, and metrics). It is also served as-is at `{Base URL}/agents_example.md`, so an agent can fetch it directly before making any API calls.
 
 ## Base Configuration
 - **Base URL**: `https://otel.k6n.net`