feat: add trending/leaderboard for collections

New collection_favorites table lets users star public collections.
/collections/explore mirrors the recipe explore page: trending (star
count in last 7 days) and recently-added public collections. Linked
from the "Discover" button on the collections page.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/db/src/schema/social.ts

@@ -6,6 +6,7 @@ import {
   boolean,
   pgEnum,
   index,
+  uniqueIndex,
 } from "drizzle-orm/pg-core";
 import { relations } from "drizzle-orm";
 import { users } from "./users";
@@ -72,6 +73,15 @@ export const collectionRecipes = pgTable("collection_recipes", {
   addedAt: timestamp("added_at").notNull().defaultNow(),
 });
 
+export const collectionFavorites = pgTable("collection_favorites", {
+  userId: text("user_id").notNull().references(() => users.id, { onDelete: "cascade" }),
+  collectionId: text("collection_id").notNull().references(() => collections.id, { onDelete: "cascade" }),
+  createdAt: timestamp("created_at").notNull().defaultNow(),
+}, (t) => [
+  index("collection_favorites_collection_idx").on(t.collectionId),
+  uniqueIndex("collection_favorites_user_collection_idx").on(t.userId, t.collectionId),
+]);
+
 export const cookingHistory = pgTable("cooking_history", {
   id: text("id").primaryKey(),
   userId: text("user_id").notNull().references(() => users.id, { onDelete: "cascade" }),
@@ -117,6 +127,11 @@ export const collectionRecipesRelations = relations(collectionRecipes, ({ one })
   recipe: one(recipes, { fields: [collectionRecipes.recipeId], references: [recipes.id] }),
 }));
 
+export const collectionFavoritesRelations = relations(collectionFavorites, ({ one }) => ({
+  collection: one(collections, { fields: [collectionFavorites.collectionId], references: [collections.id] }),
+  user: one(users, { fields: [collectionFavorites.userId], references: [users.id] }),
+}));
+
 export const collectionMemberRoleEnum = pgEnum("collection_member_role", ["viewer", "editor"]);
 
 export const collectionMembers = pgTable("collection_members", {