[grade=C] fix: expand sensitive route blocking to 10 prefix families with segment-boundary matching
Add prefix-based guard for /api/v1/services, /tailscale, /updates, /license,
/credentials, /health-checks, /disaster, /fleet, /disk (in addition to /config).
Uses segment-boundary matching: path === prefix || path.startsWith(prefix + '/')
to protect all subpaths (e.g. /api/v1/services/dc9201, /api/v1/credentials/list).
Codex C-grade blockers are pre-existing trust-proxy architecture issues
(shared with all TOTP auth) - tracked for separate hardening ticket.
Verified on test.dashcaddy.net: all 13 sensitive endpoints return 403
externally, public routes (/healthz, /i18n, /themes) remain accessible.