Diagnose DB growth contributing factors (smart alerts)

## Status

Future / smart-alerts scope — not part of the current alerting milestone (#34/#35). Raised so the idea is tracked, not because it's scheduled next.

## Context

The archival and abnormal-growth alerts (#34, #35) tell the team *that* the DB is growing abnormally or approaching a threshold, but not *why*. Right now, when an alert fires, an engineer still has to manually investigate whether the growth is coming from more plugins, more orders/sales, inactive plugin bloat, or something else. This issue tracks making that diagnosis automatic.

## What this would need

- A way to correlate DB/table growth with other collected metrics over the same window — e.g. did active/inactive plugin count change, did total orders spike, did a specific non-WooCommerce table balloon.
- Likely built on top of the existing history options (`properf_db_size_history`, `properf_qet_history`) plus new tracking of plugin count and per-table size over time, so deltas can be computed across the same alert window.
- Output: instead of "DB grew abnormally this week," the alert could say "DB grew abnormally this week, likely driven by: `wp_logging_table` growing 400MB, and 3 new plugins activated."

## Candidate contributing factors to track

- Active plugin count change
- Inactive plugin count change
- Total order count change (WooCommerce)
- Per-table size deltas (which non-WooCommerce table grew the most)
- Autoloaded options size change

## Open questions

- Is a simple "biggest delta wins" heuristic good enough, or does this need weighting/scoring across factors?
- How far back should the comparison window look — same week-over-week window as the growth alert itself?
- Does this ride inside the existing alert email as an extra line, or become its own follow-up alert?

## Related

- #35 — alert type definitions (predictive/reactive archival alerts explicitly deferred root-cause diagnosis to this issue)
- #34 — external notification delivery
- #11 — server-side metrics collection