feat(metrics): SQLite persistence + chart fixes (no dots, peak label, min/avg/max in title)

- Add modernc.org/sqlite dependency; write every sample to
  /appdata/bee/metrics.db (WAL mode, prune to 24h on startup)
- Pre-fill ring buffers from last 120 DB rows on startup so charts
  survive service restarts
- Ticker changed 3s→1s; chart JS refresh will be set to 2s (lag ≤3s)
- Add GET /api/metrics/export.csv for full history download
- Chart rendering: SymbolNone (no dots), right padding=80px so peak
  mark line label is not clipped, min/avg/max appended to chart title

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

audit/go.sum

@@ -8,11 +8,30 @@ github.com/go-analyze/charts v0.5.26 h1:rSwZikLQuFX6cJzwI8OAgaWZneG1kDYxD857ms00
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