Log a masked account summary on startup

[vgpastor]

Goal / Objetivo

EN: On startup, log a one-line summary of the accounts that were loaded
(channel + emergency slug + masked identifier), so operators can confirm the
right config booted — without printing any secret.

ES: Al arrancar, loguear un resumen de una línea de las cuentas cargadas
(canal + slug de emergencia + identificador enmascarado), para que los operadores
confirmen que arrancó la config correcta — sin imprimir ningún secreto.

Where / Dónde

src/index.ts (bootstrap), after accounts.json is loaded and validated.

Acceptance / Aceptación

• One log line listing each account by id, channel, emergencySlug.
• Tokens/phone numbers are masked (e.g. last 4 chars only) or omitted entirely.
• A test asserting no secret substring appears in the output.

Good first issue — small and well-scoped. Mind the "no secrets in logs" rule.