feat(backup): GFS retention policy (pure)

14 daily + 4 weekly (Mondays) + 6 monthly (1st). Future-dated files
preserved (clock skew). Unrecognized filenames ignored (no delete).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main/services/backupRotation.ts

@@ -0,0 +1,75 @@
+const BACKUP_FILENAME_REGEX = /^inkling-(\d{4}-\d{2}-\d{2})\.sqlite$/;
+const ONE_DAY_MS = 24 * 60 * 60 * 1000;
+const DAILY_WINDOW_DAYS = 14;
+const WEEKLY_WINDOW_COUNT = 4;
+const MONTHLY_WINDOW_COUNT = 6;
+
+export function parseBackupFilename(name: string): string | null {
+  const m = BACKUP_FILENAME_REGEX.exec(name);
+  if (!m) return null;
+  const iso = m[1]!;
+  const d = new Date(iso + 'T00:00:00Z');
+  if (Number.isNaN(d.getTime()) || d.toISOString().slice(0, 10) !== iso) return null;
+  return iso;
+}
+
+export interface RetentionResult {
+  keep: string[];
+  remove: string[];
+}
+
+function startOfDayUtc(d: Date): Date {
+  const x = new Date(d);
+  x.setUTCHours(0, 0, 0, 0);
+  return x;
+}
+
+function isWithinDailyWindow(fileDate: Date, now: Date): boolean {
+  const today = startOfDayUtc(now);
+  const oldest = new Date(today.getTime() - (DAILY_WINDOW_DAYS - 1) * ONE_DAY_MS);
+  return fileDate >= oldest && fileDate <= today;
+}
+
+function isWithinWeeklyWindow(fileDate: Date, now: Date): boolean {
+  // UTC-based Monday detection. UTCDay: 0=Sun, 1=Mon..6=Sat
+  if (fileDate.getUTCDay() !== 1) return false;
+  const today = startOfDayUtc(now);
+  // Anchor to the most recent Monday on/before today, then reach back
+  // WEEKLY_WINDOW_COUNT * 7 days. This keeps 4 Mondays before the anchor.
+  const dayOfWeek = today.getUTCDay(); // 0=Sun..6=Sat
+  const daysSinceMonday = (dayOfWeek + 6) % 7; // Mon=0, Sun=6
+  const lastMonday = new Date(today.getTime() - daysSinceMonday * ONE_DAY_MS);
+  const oldest = new Date(lastMonday.getTime() - WEEKLY_WINDOW_COUNT * 7 * ONE_DAY_MS);
+  return fileDate >= oldest && fileDate <= today;
+}
+
+function isWithinMonthlyWindow(fileDate: Date, now: Date): boolean {
+  if (fileDate.getUTCDate() !== 1) return false;
+  const today = startOfDayUtc(now);
+  // months ago: difference in calendar months
+  const monthsAgo =
+    (today.getUTCFullYear() - fileDate.getUTCFullYear()) * 12 +
+    (today.getUTCMonth() - fileDate.getUTCMonth());
+  return monthsAgo >= 0 && monthsAgo < MONTHLY_WINDOW_COUNT;
+}
+
+export function applyGfsRetention(filenames: string[], now: Date): RetentionResult {
+  const keep: string[] = [];
+  const remove: string[] = [];
+  for (const name of filenames) {
+    const iso = parseBackupFilename(name);
+    if (iso === null) continue; // unrecognized — ignore (no-op)
+    const fileDate = new Date(iso + 'T00:00:00Z');
+    if (fileDate > startOfDayUtc(now)) {
+      keep.push(name); // future-dated — clock skew safety
+      continue;
+    }
+    const survives =
+      isWithinDailyWindow(fileDate, now) ||
+      isWithinWeeklyWindow(fileDate, now) ||
+      isWithinMonthlyWindow(fileDate, now);
+    if (survives) keep.push(name);
+    else remove.push(name);
+  }
+  return { keep, remove };
+}