Add server-side Collection API service (ServerSideEventSender)
New autowired service that POSTs events to the PA Collection API using the site ID and collect domain from pianoanalytics.settings.
- Content-Type: text/plain per PA Collection API spec
- Auto-enriches properties with device_timestamp_utc and event_collection_platform
- Forwards browser context headers (User-Agent, Referer, X-Forwarded-For) for device and geo analysis
- resolveVisitorId() reads PA cookies (_pcid, pa_vid, atuserid) with sha256(IP) fallback
- extractBrowserHeaders() extracts relevant headers from request
- 2s timeout, catches all exceptions (never breaks callers)
- 6 kernel tests
Closes #3579450