feat: default to average downsampling

Average consolidation makes every plotted point the true arithmetic mean of
its interval, which is what the Ganglia-style plot appearance depends on.
LTTB deliberately keeps extremes instead, so a plot downsampled with it
cannot be read as interval means.

resolveResampleAlgo used to return an empty string when neither the user nor
the config had chosen an algorithm, and cc-lib's GetResampler maps that to
LTTB. Route every fallback through config.ResampleAlgo() and ship "average"
as the generated and example default.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

cmd/cc-backend/init.go

@@ -25,7 +25,7 @@ const configString = `
     "short-running-jobs-duration": 300,
     "resampling": {
       "default-policy": "medium",
-      "default-algo": "lttb"
+      "default-algo": "average"
     },
     "api-allowed-ips": [
       "*"