VAPID config and push subscription API (Feature 19) (#153)

Co-authored-by: orchestrator-agent <20+orchestrator-agent@code.cubecraftcreations.com>

migrations/queries/push_subscriptions.sql

@@ -0,0 +1,20 @@
+-- name: UpsertPushSubscription :one
+-- Stores a browser's push subscription, keyed by its endpoint. Subscribing
+-- an endpoint already stored replaces its keys and user agent and clears
+-- its failure count, keeping its id, created_at and last_success_at.
+INSERT INTO push_subscriptions (
+	id, endpoint, p256dh, auth, user_agent, failure_count, created_at, updated_at
+) VALUES (
+	?, ?, ?, ?, ?, 0, ?, ?
+)
+ON CONFLICT (endpoint) DO UPDATE SET
+	p256dh = excluded.p256dh,
+	auth = excluded.auth,
+	user_agent = excluded.user_agent,
+	failure_count = 0,
+	updated_at = excluded.updated_at
+RETURNING *;
+
+-- name: DeletePushSubscription :execrows
+DELETE FROM push_subscriptions
+WHERE endpoint = ?;