refactor(cron): rename scheduler feature to cron

Rename the recurring-jobs feature scheduler->cron to disambiguate from the
legacy ScheduledRun system (/api/scheduled), which is left untouched:

- ScheduledJob->CronJob, SchedulerService->CronService
- /api/scheduler/jobs -> /api/cron/jobs; SSE scheduler:* -> cron:*
- state keys cronJobs/cronJobRuns
- files moved to src/cron/, cron-routes.ts, cron-port.ts, types/cron.ts
- frontend cron-ui.js, #cronModal, menu "Cron"
- docs moved to docs/cron-discovery.md + docs/cron-build-brief.md, README guides
- new tests: cron-service.test.ts, cron-time.test.ts

Green: tsc, lint, frontend-syntax, format, 30 cron + 9 legacy scheduled-runs tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PmvZR12aX2v8K7YhqxPUAU

src/cron/cron-time.ts

@@ -0,0 +1,82 @@
+/**
+ * @fileoverview Pure next-run-time calculations for the cron.
+ *
+ * All functions are pure and take an explicit `after` timestamp (epoch ms) so
+ * they are deterministic and unit-testable. Times use the SERVER'S LOCAL
+ * timezone for v0.1 (per the build brief) — daily/weekly wall-clock times are
+ * interpreted via the host's local time.
+ */
+
+import type { CronJob } from '../types/cron.js';
+
+/** Parse an 'HH:MM' (24-hour) string into hours/minutes, or null if invalid. */
+export function parseHHMM(value: string | undefined): { hours: number; minutes: number } | null {
+  if (!value) return null;
+  const m = /^(\d{1,2}):(\d{2})$/.exec(value.trim());
+  if (!m) return null;
+  const hours = Number(m[1]);
+  const minutes = Number(m[2]);
+  if (hours < 0 || hours > 23 || minutes < 0 || minutes > 59) return null;
+  return { hours, minutes };
+}
+
+/**
+ * Returns the epoch-ms timestamp for `hours:minutes` (local time) on the day of
+ * `base`, shifted by `dayOffset` days.
+ */
+function atLocalTime(base: number, hours: number, minutes: number, dayOffset: number): number {
+  const d = new Date(base);
+  d.setHours(hours, minutes, 0, 0);
+  d.setDate(d.getDate() + dayOffset);
+  return d.getTime();
+}
+
+/**
+ * Compute the next fire time strictly relevant to `after`, or null if the job
+ * has no future run (e.g. a completed one-time job, or invalid config).
+ *
+ * For `once`, returns the absolute `runAt` (even if already in the past, so a
+ * missed one-time job still fires once) until it has `completedOnce`.
+ */
+export function computeNextRunAt(job: CronJob, after: number): number | null {
+  switch (job.scheduleType) {
+    case 'once': {
+      if (job.completedOnce) return null;
+      return typeof job.runAt === 'number' ? job.runAt : null;
+    }
+    case 'interval': {
+      const minutes = job.intervalMinutes;
+      if (!minutes || minutes <= 0) return null;
+      return after + minutes * 60_000;
+    }
+    case 'daily': {
+      const t = parseHHMM(job.dailyTime);
+      if (!t) return null;
+      let next = atLocalTime(after, t.hours, t.minutes, 0);
+      if (next <= after) next = atLocalTime(after, t.hours, t.minutes, 1);
+      return next;
+    }
+    case 'weekly': {
+      const t = parseHHMM(job.weeklyTime);
+      if (!t) return null;
+      const days = (job.weeklyDays ?? []).filter((d) => d >= 0 && d <= 6);
+      if (days.length === 0) return null;
+      for (let offset = 0; offset <= 7; offset++) {
+        const cand = atLocalTime(after, t.hours, t.minutes, offset);
+        if (cand > after && days.includes(new Date(cand).getDay())) return cand;
+      }
+      return null;
+    }
+    default:
+      return null;
+  }
+}
+
+/**
+ * Duplicate-launch guard key: identifies a specific due time for a job. The
+ * cron records the key it last consumed so an overlapping or restarted
+ * loop will not launch the same due time twice.
+ */
+export function dueKeyFor(jobId: string, fireTime: number): string {
+  return `${jobId}:${fireTime}`;
+}