feat(backup): atomic SQLite snapshot to inkling-YYYY-MM-DD.sqlite

KST date filename, tmp+rename atomic write, mkdir on demand.

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

src/main/services/BackupService.ts

@@ -0,0 +1,55 @@
+import type Database from 'better-sqlite3';
+import { mkdir, rename, stat, readdir, unlink } from 'node:fs/promises';
+import { join } from 'node:path';
+import { applyGfsRetention } from './backupRotation.js';
+
+const KST_OFFSET_MS = 9 * 60 * 60 * 1000;
+
+function toKstDateKey(d: Date): string {
+  const k = new Date(d.getTime() + KST_OFFSET_MS);
+  return k.toISOString().slice(0, 10);
+}
+
+export interface SnapshotResult {
+  path: string;
+  bytes: number;
+}
+
+export interface RotateResult {
+  kept: string[];
+  removed: string[];
+}
+
+export class BackupService {
+  constructor(
+    private db: Database.Database,
+    private backupDir: string,
+    private now: () => Date = () => new Date()
+  ) {}
+
+  async snapshot(): Promise<SnapshotResult> {
+    await mkdir(this.backupDir, { recursive: true });
+    const dateKey = toKstDateKey(this.now());
+    const finalPath = join(this.backupDir, `inkling-${dateKey}.sqlite`);
+    const tmpPath = `${finalPath}.tmp`;
+    await this.db.backup(tmpPath);
+    await rename(tmpPath, finalPath);
+    const st = await stat(finalPath);
+    return { path: finalPath, bytes: st.size };
+  }
+
+  async rotate(): Promise<RotateResult> {
+    let entries: string[];
+    try {
+      entries = await readdir(this.backupDir);
+    } catch (e: unknown) {
+      if ((e as NodeJS.ErrnoException).code === 'ENOENT') return { kept: [], removed: [] };
+      throw e;
+    }
+    const decision = applyGfsRetention(entries, this.now());
+    for (const name of decision.remove) {
+      await unlink(join(this.backupDir, name));
+    }
+    return { kept: decision.keep, removed: decision.remove };
+  }
+}