feat: enhance limits

migrations/0042_user_limit_overrides.up.sql

@@ -0,0 +1,15 @@
+-- Per-user save-limit overrides, set by admins from the panel. NULL means
+-- "use the plan default" (free: 1 bookmark, 5 public + 5 private collections;
+-- premium: 5 bookmarks, unlimited collections). A non-NULL value caps that
+-- resource for this specific user, regardless of plan.
+ALTER TABLE users
+    ADD COLUMN IF NOT EXISTS bookmark_limit           INT,
+    ADD COLUMN IF NOT EXISTS public_collection_limit  INT,
+    ADD COLUMN IF NOT EXISTS private_collection_limit INT;
+
+ALTER TABLE users
+    ADD CONSTRAINT users_limit_overrides_nonneg CHECK (
+        (bookmark_limit           IS NULL OR bookmark_limit           >= 0) AND
+        (public_collection_limit  IS NULL OR public_collection_limit  >= 0) AND
+        (private_collection_limit IS NULL OR private_collection_limit >= 0)
+    );