feat: fix dashboard duplicates, add metrics backlog (funnel, TTFB, adoption, bot instrumentation)

Wrap all dashboard stat/timeseries expressions in max()/sum() to
eliminate duplicate series from multiple Prometheus scrape targets.

Add 8 new metrics across web-api and bot crates: onboarding funnel,
time-to-first-bouncer, feature adoption, connected users 5m,
per-network deliveries, bot script runs/duration/errors, and bot
background gauges. Add 10 new Grafana panels including HTTP error
rates and storage growth prediction.

Signed-off-by: Josephine Pfeiffer <hi@josie.lol>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Josephine Pfeiffer <hi@josie.lol>