Add logToSyslog option for journald/syslog logging

[anpryl]

Summary

Add a logToSyslog boolean option (default false) that sends all nginx log output to syslog via the unix socket /dev/log instead of files. On systemd-based systems like NixOS, journald captures syslog messages automatically — providing automatic rotation, structured querying via journalctl, and eliminating the need for logrotate configuration.

Changes

• New option services.lancache.logToSyslog — when true, all 6 nginx log directives use syslog:server=unix:/dev/log,facility=local6,tag=<tag> instead of file paths
• Changed logPrefix type from types.str to types.nullOr types.str — allows omitting it when syslog is enabled
• Added assertion — ensures at least one logging method (logPrefix or logToSyslog) is configured
• Refactored log targets — extracted logTarget helper function to reduce duplication across the 6 log directives
• Per-context syslog tags for easy filtering:
  • lancache — primary cache engine
  • lancache_upstream — upstream redirect handler
  • lancache_stream — TLS SNI passthrough

Backward compatibility

• logToSyslog defaults to false — existing configs are unaffected
• nullOr str is a superset of str — every existing logPrefix value remains valid
• File-based log filenames are preserved exactly (access.log, upstream-access.log, stream-access.log, etc.)

Usage

services.lancache = {
  enable = true;
  logToSyslog = true;
  # logPrefix is no longer needed
};

Filter logs with:

journalctl _COMM=nginx --grep="lancache:"
journalctl _COMM=nginx --grep="lancache_upstream:"
journalctl _COMM=nginx --grep="lancache_stream:"

Test plan

• Build NixOS configuration with logToSyslog = true — succeeds
• Deploy to NixOS server — nginx starts without errors
• Verify syslog entries appear in journald after HTTP request
• Verify backward compatibility: logToSyslog = false with logPrefix set produces identical nginx config to upstream