Add per-payer and per-member custom split amounts with Equally, Custom and
Ticket split modes on top of the existing equal split, keep legacy "paid by"
expenses working, and document the modes in the Budget Tracking wiki page.
Adds the complete TREK documentation wiki covering installation,
trip planning, admin panel, MCP/AI integration, addons, and operations.
Also fixes encrypt-at-rest gaps: mapbox_access_token, Synology
credentials, per-user webhook/ntfy tokens, and photo passphrases
are now rotated by migrate-encryption.ts and stored encrypted via
settingsService.