Scheduled jobs

KickJS's @Cron: any service lists its jobs, and the App schedules them once it is listening.

pub const MaintenanceJobs = struct {
    reports: *ReportService,

    pub const crons = .{
        .{ .expression = "*/5 * * * *", .handler = "purge", .name = "purge-sessions" },
        .{ .expression = "0 2 * * *", .handler = "nightly", .timezone = "Africa/Nairobi" },
        .{ .expression = "0 * * * *", .handler = "warm", .run_on_init = true },
    };

    /// Asked on every tick and manual run.
    pub fn cronEnabled(self: *MaintenanceJobs, name: []const u8) bool { … }

    pub fn nightly(self: *MaintenanceJobs, run: kzig.cron.Run) !void {
        try self.reports.generate(run.arena);
    }
};

Expressions have 5 fields or 6 (seconds first), with names, ranges, steps and lists; a bad one fails zig build. Timezones come from the system's zoneinfo (CRON_ZONEINFO). A run while the previous is still going is skipped unless .overlap = true. Across a daylight-saving change, a local time that doesn't exist is skipped, and in a repeated hour only jobs with * hours run again. CRON_ENABLED=false schedules nothing (one replica of several, say). Inject *kzig.cron.Jobs to list jobs, get their next run or run one now; a failure reaches adapters' onCronError(err, info). Shutdown stops the schedule and waits for runs in flight. task-api's crons module is the archive's cron-jobs-api.