follow-up: #3584516 add configurable visit storage setting (cache clear)

Add builtin_stats setting with two-level UI:

• Enable visit tracking (checkbox): controls middleware
• Enable built-in statistics (checkbox, nested): controls DB subscriber and stats visibility

ServiceProvider removes services at compile time:

• tracking off: middleware + DB subscriber removed
• stats off: DB subscriber removed, event still dispatches

VisitTracker::supportsStats() reads config to hide stats tab when built-in statistics are disabled.

Closes #3584516