feat: add web push and sadhna tracker

migrations/0027_create_push_subscriptions.up.sql

@@ -0,0 +1,18 @@
+-- Web Push subscriptions. One row per browser/device push endpoint a user
+-- has granted. The endpoint is globally unique (it's a per-subscription URL
+-- at the push service), so re-subscribing the same browser upserts. p256dh
+-- and auth are the subscription's encryption keys (base64url). Dead
+-- endpoints (404/410 from the push service) are pruned on send.
+CREATE TABLE IF NOT EXISTS push_subscriptions (
+    id           UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id      UUID         NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    endpoint     TEXT         NOT NULL UNIQUE,
+    p256dh       TEXT         NOT NULL,
+    auth         TEXT         NOT NULL,
+    user_agent   TEXT         NOT NULL DEFAULT '',
+    created_at   TIMESTAMPTZ  NOT NULL DEFAULT now(),
+    last_used_at TIMESTAMPTZ  NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS push_subscriptions_user_idx
+    ON push_subscriptions (user_id);