Implement database administration

Add routes and admin panel elements for creating a database backup,
restoring from a backup, deleting the existing schedule, and replacing
the database with the demo schedule.  These server as crude ways to
manage the data stored in the system.

server/api/admin/database-import.post.ts

@@ -0,0 +1,44 @@
+import { readNextSessionId, readNextUserId, readSessions, type ServerSession, type ServerUser, writeNextSessionId, writeNextUserId, writeSchedule, writeSessions, writeSubscriptions, writeUsers } from "~/server/database";
+import type { ApiSchedule, ApiSubscription } from "~/shared/types/api";
+
+export default defineEventHandler(async (event) => {
+	await requireServerSessionWithAdmin(event);
+	const formData = await readMultipartFormData(event);
+	let snapshot: undefined | {
+		nextUserId: number,
+		users: ServerUser[],
+		nextSessionId: number,
+		sessions: ServerSession[],
+		subscriptions: ApiSubscription[],
+		schedule: ApiSchedule,
+	};
+	for (const part of formData ?? []) {
+		if (part.name === "snapshot") {
+			snapshot = JSON.parse(part.data.toString("utf-8"));
+		}
+	}
+	if (!snapshot) {
+		throw createError({
+			statusCode: 400,
+			statusMessage: "Bad Request",
+			message: "snapshot missing."
+		});
+	}
+
+	const currentNextUserId = await readNextUserId();
+	await writeNextUserId(Math.max(currentNextUserId, snapshot.nextUserId));
+	await writeUsers(snapshot.users);
+	const currentNextSessionId = await readNextSessionId();
+	await writeNextSessionId(Math.max(currentNextSessionId, snapshot.nextSessionId));
+	const currentSessions = new Map((await readSessions()).map(session => [session.id, session]));
+	await writeSessions(snapshot.sessions.filter(session => {
+		const current = currentSessions.get(session.id);
+		// Only keep sessions that match the account id in both sets to avoid
+		// resurrecting deleted sessions. This will still cause session cross
+		// pollution if a snapshot from another instance is loaded here.
+		return current && current.account.id === session.account.id;
+	}));
+	await writeSubscriptions(snapshot.subscriptions);
+	await writeSchedule(snapshot.schedule);
+	await sendRedirect(event, "/");
+})