added sharing and subscriptions

backend/migrations/0006_share.sql

@@ -0,0 +1,6 @@
+-- Public read-only sharing for lists.
+-- NULL share_token = private (default). A non-NULL token is an unguessable
+-- secret: anyone holding it can view the list read-only at /api/shared/{token},
+-- no account required. Revoking (unshare) sets it back to NULL.
+ALTER TABLE lists
+    ADD COLUMN share_token TEXT UNIQUE;

backend/migrations/0007_subscriptions.sql

@@ -0,0 +1,31 @@
+-- Subscriptions: a logged-in user follows someone else's shared list or a
+-- single shared item, to receive the same price-drop emails the owner gets.
+-- Exactly one of list_id / item_id is set per row. A list subscription
+-- implicitly covers every item on that list — present and future — expanded
+-- at notify time rather than materialised per item.
+CREATE TABLE subscriptions (
+    id         UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id    UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    list_id    UUID REFERENCES lists(id) ON DELETE CASCADE,
+    item_id    UUID REFERENCES items(id) ON DELETE CASCADE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    CONSTRAINT subscription_target_exactly_one
+        CHECK ((list_id IS NOT NULL) <> (item_id IS NOT NULL))
+);
+
+-- One subscription per (user, list) and per (user, item).
+CREATE UNIQUE INDEX subscriptions_user_list
+    ON subscriptions (user_id, list_id) WHERE list_id IS NOT NULL;
+CREATE UNIQUE INDEX subscriptions_user_item
+    ON subscriptions (user_id, item_id) WHERE item_id IS NOT NULL;
+
+-- Per-subscriber, per-item de-dupe latch for target-price alerts — the
+-- subscriber-side mirror of items.notified_at (which latches the owner).
+-- Present = already announced this drop; absent = armed. Cleared when the
+-- price climbs back above the item's target.
+CREATE TABLE subscription_notify (
+    user_id     UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    item_id     UUID NOT NULL REFERENCES items(id) ON DELETE CASCADE,
+    notified_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (user_id, item_id)
+);