Prepare for an eventual frontend

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

backend/db/init.sql

@@ -0,0 +1,20 @@
+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
+);