Prepare for an eventual frontend

Move the backend code into a backend/ directory, and create a frontend/
directory containing an empty sveltekit project.

db/init.sql

@@ -1,20 +0,0 @@
-CREATE TABLE datasets (
-    id          SERIAL PRIMARY KEY,
-    name        TEXT NOT NULL,
-    description TEXT
-);
-
-CREATE TABLE locations (
-    id   SERIAL PRIMARY KEY,
-    name TEXT,
-    lat  DOUBLE PRECISION NOT NULL,
-    lon  DOUBLE PRECISION NOT NULL
-);
-
-CREATE TABLE records (
-    id          SERIAL PRIMARY KEY,
-    dataset_id  INTEGER NOT NULL REFERENCES datasets(id),
-    location_id INTEGER REFERENCES locations(id),
-    timestamp   TIMESTAMPTZ NOT NULL DEFAULT now(),
-    data        JSONB
-);