Bun SQLite — Local-First App Backend

## Task
Local-first backend using Bun's native SQLite bindings.

## Requirements
- Runtime: Bun (not Node.js)
- Database: Bun's built-in bun:sqlite
- HTTP: Bun.serve()
- Zero external dependencies

## Specifications
```typescript
import { Database } from "bun:sqlite";

// Features:
// 1. WAL mode for concurrent reads during writes
// 2. Prepared statements (reusable, faster)
// 3. Typed query results with generics
// 4. Migration system (SQL files in /migrations)
// 5. Full-text search using FTS5
// 6. Auto-backup every hour

// API:
// GET    /api/notes          → list (with FTS search via ?q=)
// POST   /api/notes          → create
// GET    /api/notes/:id      → read
// PUT    /api/notes/:id      → update
// DELETE /api/notes/:id      → delete

// Performance targets:
// - Simple GET: <0.5ms
// - FTS search: <2ms for 100K records
// - Throughput: 50K requests/sec on single core
```

## Implementation Notes
1. Enable WAL: `db.exec("PRAGMA journal_mode = WAL")`
2. Use `db.prepare()` for all repeated queries — 10x faster than `db.query()`
3. FTS5: create virtual table, sync with triggers on main table
4. Migration runner: read SQL files sorted by timestamp, track applied in _migrations table
5. Transactions for multi-step operations
6. Graceful shutdown: checkpoint WAL before exit