chore: replace better-sqlite3 with bun:sqlite

[sasagar]

Closes #166

Summary

C++ ネイティブモジュール better-sqlite3 を Bun 組み込みの bun:sqlite に置換。CI から Node.js セットアップを撤廃し、ネイティブビルド依存（python / build-essential）も不要に。

Why

• better-sqlite3 は Bun では動作せず、CI で Node.js 24 を追加セットアップする運用だった
• ネイティブモジュールはサプライチェーン攻撃の表面積でもある（pre-built binary、build-time script）
• Bun の bun:sqlite + drizzle-orm/bun-sqlite で同等機能を提供できる

Changes

Driver swap (packages/backend/src/db/index.ts)

• drizzle-orm/better-sqlite3 → drizzle-orm/bun-sqlite
• better-sqlite3 → bun:sqlite（Bun 組み込み）
• sqlite.pragma("journal_mode = WAL") → sqlite.exec("PRAGMA journal_mode = WAL")
• SqliteDatabase 型を共通 export（D1 型もこれに合流）

Repository (22 files)

• BetterSQLite3Database → BunSQLiteDatabase（型エイリアスのみ、ロジック変更なし）

Tests

• src/tests/node/sqlite-repositories.test.ts（vitest 経由）を削除
• src/tests/integration/sqlite-repositories.test.ts に bun:test + bun:sqlite で再実装（28 テスト、全 pass）

Dependencies

• 削除: better-sqlite3, @types/better-sqlite3, vitest
• test:sqlite script: npx vitest run …node/… → bun test …integration/…

CI (.github/workflows/ci.yml)

• sqlite-tests job から actions/setup-node ステップを削除
• bunx vitest → bun test

Test plan

ローカル検証済み（macOS, Bun 1.3.13）：

• bun run typecheck (backend + frontend) — pass
• bun run lint — 0 warnings, 0 errors (576 files)
• bun test src/tests/unit/ — 1012 pass / 0 fail
• bun test src/tests/integration/ — 70 pass / 0 fail（うち 28 が SQLite）
• bun run build (backend) — 6.65 MB, 1966 modules
• node_modules/better-sqlite3, vitest, @types/better-sqlite3 — すべて不在を確認

Compatibility

• SQLite ファイルフォーマットは完全互換（既存 DB をそのまま使える）
• bun:sqlite API は better-sqlite3 とほぼ互換、Drizzle 抽象化で吸収
• D1 (Cloudflare Workers) パスは型のみ変更、実行時挙動は不変

Conflicts with PR #164

.github/workflows/ci.yml の sqlite-tests job が PR #164（hardening: Actions SHA 固定）と重なります。マージ順序により：

• 先に chore: harden CI/CD against supply-chain attacks #164 がマージされた場合、このブランチを rebase して setup-node 削除のみ残します
• 後に chore: harden CI/CD against supply-chain attacks #164 がマージされた場合、chore: harden CI/CD against supply-chain attacks #164 側で sqlite-tests から setup-node を削除した状態でマージされます

どちらにせよ簡単な競合解決で済みます。

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 24791bd6-0736-41a3-8577-241c15f02326

📥 Commits

Reviewing files that changed from the base of the PR and between 3c76a5c and 865a259.

⛔ Files ignored due to path filters (1)

• bun.lock is excluded by !**/*.lock

📒 Files selected for processing (28)

• .github/workflows/ci.yml
• packages/backend/package.json
• packages/backend/src/db/index.ts
• packages/backend/src/db/schema/sqlite.ts
• packages/backend/src/repositories/sqlite/SqliteCustomEmojiRepository.ts
• packages/backend/src/repositories/sqlite/SqliteDriveFileRepository.ts
• packages/backend/src/repositories/sqlite/SqliteDriveFolderRepository.ts
• packages/backend/src/repositories/sqlite/SqliteFollowRepository.ts
• packages/backend/src/repositories/sqlite/SqliteInstanceBlockRepository.ts
• packages/backend/src/repositories/sqlite/SqliteInstanceSettingsRepository.ts
• packages/backend/src/repositories/sqlite/SqliteInvitationCodeRepository.ts
• packages/backend/src/repositories/sqlite/SqliteModerationAuditLogRepository.ts
• packages/backend/src/repositories/sqlite/SqliteNoteRepository.ts
• packages/backend/src/repositories/sqlite/SqliteNotificationRepository.ts
• packages/backend/src/repositories/sqlite/SqliteOAuthAccountRepository.ts
• packages/backend/src/repositories/sqlite/SqlitePasskeyChallengeRepository.ts
• packages/backend/src/repositories/sqlite/SqlitePasskeyCredentialRepository.ts
• packages/backend/src/repositories/sqlite/SqliteReactionRepository.ts
• packages/backend/src/repositories/sqlite/SqliteRemoteInstanceRepository.ts
• packages/backend/src/repositories/sqlite/SqliteRoleAssignmentRepository.ts
• packages/backend/src/repositories/sqlite/SqliteRoleRepository.ts
• packages/backend/src/repositories/sqlite/SqliteScheduledNoteRepository.ts
• packages/backend/src/repositories/sqlite/SqliteSessionRepository.ts
• packages/backend/src/repositories/sqlite/SqliteUserReportRepository.ts
• packages/backend/src/repositories/sqlite/SqliteUserRepository.ts
• packages/backend/src/repositories/sqlite/SqliteUserWarningRepository.ts
• packages/backend/src/tests/integration/sqlite-repositories.test.ts
• packages/backend/src/tests/node/sqlite-repositories.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/issue-166-bun-sqlite-migration

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[sasagar]

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