feat: add runtime statistics to healthcheck endpoint

Add the following counters to the healthcheck JSON response:

- sessions: current active session count (from DB)
- clients: current connected client count (from DB)
- queuedLines: total entries in client output queues (from DB)
- channels: current channel count (from DB)
- connectionsSinceBoot: total client connections since server start
- sessionsSinceBoot: total sessions created since server start
- messagesSinceBoot: total PRIVMSG/NOTICE messages since server start

Implementation:
- New internal/stats package with atomic counters for boot-scoped metrics
- New DB queries GetClientCount and GetQueueEntryCount
- Healthcheck.Healthcheck() now accepts context for DB queries
- Counter increments in session creation, registration, login, and messaging
- Stats tracker wired via Uber fx dependency injection
- Unit tests for stats package (100% coverage) and integration tests
- README updated with full healthcheck response documentation

closes #74

internal/handlers/healthcheck.go

@@ -12,7 +12,7 @@ func (hdlr *Handlers) HandleHealthCheck() http.HandlerFunc {
 		writer http.ResponseWriter,
 		request *http.Request,
 	) {
-		resp := hdlr.hc.Healthcheck()
+		resp := hdlr.hc.Healthcheck(request.Context())
 		hdlr.respondJSON(writer, request, resp, httpStatusOK)
 	}
 }