feat: auto-HTTPS and graceful shutdown fix

[GaIsBAX]

Changes

fix: graceful shutdown

Shutdown was called with an already-cancelled context, so context.WithTimeout(ctx, 10s) produced an immediately-cancelled child context - the server never got its 10-second grace window and always did a hard kill. Fixed by using context.Background() instead.

feat: auto-HTTPS via Let's Encrypt

When WEBHIX_TLS_DOMAIN is set, Webhix handles TLS itself - no nginx or Caddy required.

• Listens on :443 (HTTPS) and :80 (HTTP → HTTPS redirect + ACME challenge)
• Certificate is obtained and renewed automatically via Let's Encrypt
• WEBHIX_ADDR is ignored when TLS is active
• WEBHIX_TLS_CACHE_DIR controls where certs are cached (default: ./data/certs)

Usage:

WEBHIX_TLS_DOMAIN=hooks.example.com
WEBHIX_BASE_URL=https://hooks.example.com