feat: add SQLite database and persistent task schema

lib/db.ts

@@ -0,0 +1,36 @@
+import fs from "node:fs";
+import path from "node:path";
+import { DatabaseSync } from "node:sqlite";
+
+type DatabaseGlobal = typeof globalThis & {
+  plannerDatabase?: DatabaseSync;
+  plannerDatabasePath?: string;
+};
+
+const schemaPath = path.join(process.cwd(), "db", "schema.sql");
+const defaultDatabasePath = path.join(process.cwd(), "data", "planner.sqlite");
+
+export function getDatabase() {
+  const databasePath = process.env.TASKS_DB_PATH ?? defaultDatabasePath;
+  const globalDatabase = globalThis as DatabaseGlobal;
+
+  if (
+    globalDatabase.plannerDatabase &&
+    globalDatabase.plannerDatabasePath === databasePath
+  ) {
+    return globalDatabase.plannerDatabase;
+  }
+
+  fs.mkdirSync(path.dirname(databasePath), { recursive: true });
+
+  const database = new DatabaseSync(databasePath);
+  const schema = fs.readFileSync(schemaPath, "utf8");
+
+  database.exec("PRAGMA foreign_keys = ON;");
+  database.exec(schema);
+
+  globalDatabase.plannerDatabase = database;
+  globalDatabase.plannerDatabasePath = databasePath;
+
+  return database;
+}