docs(admin): document the repeater lifecycle & data cleanup settings

[MrAlders0n]

Documents the new regional admin panel (dev/admin.php) cleanup features, grouped the way the panel groups them.

What's new

A single Repeater Lifecycle & Data Cleanup section under System Settings, mirroring the panel's Repeaters & Data Integrity accordion. It opens with a lifecycle-at-a-glance table (what happens at 24h / 72h / 30d / never, and which setting drives each stage), then covers every control with a worked example:

• Stale Repeater Age — moved here from the flat settings list, with both 3× rules spelled out
• Repeater Inactive After — newly configurable per region (was hardcoded to 30 days)
• Repeater Retention / Auto-Delete — the destructive one
• Ghost Retention — what a ghost is, and why the catalog is the evidence base for pending links
• Pending Link Distance — the ghost-repeater-steals-a-distant-ID problem, and why the default is 200 km
• Stale Ping Cleanup — the grace clock, plus the Backfill Purge preview/confirm flow

Two behaviours worth calling out

Both are documented as the code actually behaves, which differs from how the in-app help text reads:

1. The retention clock runs from the last advert heard, not from when the repeater went inactive. The windows overlap on one timeline rather than running back to back — with Inactive After = 30 and Retention = 90, deletion happens at day 90, leaving 60 days in the recoverable Inactive state, not 90.
2. A sub-minimum retention value is rejected, not clamped. The setting silently reverts to Disabled rather than snapping up to the minimum.

The panel's warning box will be updated separately in the server repo to match.

Also covered

• Which settings a multi-region group controls fleet-wide vs. which stay per-region, and the group-then-region resolution order
• Pending Repeater Links alert — reading the evidence tiers, the three choices, why suppression clears itself, and the two cases where linking is refused
• Suspicious Live Sessions alert
• Three new notification events: Suspicious Flight, Pending Repeater Link, Ping Purge Cleanup Report
• Corrections to the existing Inactive status and Bypass Auto Delete descriptions, which both referenced the old hardcoded 30 days

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