fix: resolve TODO.md security/perf/test-coverage backlog

Fixes the 13-item codebase health scan backlog: wraps meal-plan
generation in a transaction, adds missing userId/GIN indexes, fixes
an IPv6-parsing gap in the webhook SSRF guard (and an identical
duplicated bug in the AI URL-import path, now consolidated onto one
implementation), paginates the collections list, dedupes the AI
recipe Zod schemas, wires up Stripe tier sync, rate-limits AI key
rotation, gets `pnpm typecheck` actually working, and adds test
coverage for the previously-untested admin/webhooks routes.

Two flagged issues (collection removeRecipeId IDOR, tier-limit race)
turned out to already be fixed/non-issues on inspection — noted in
TODO.md rather than silently dropped.

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

packages/db/src/schema/users.ts

@@ -23,6 +23,7 @@ export const users = pgTable("users", {
   username: text("username").unique(),
   role: userRoleEnum("role").notNull().default("user"),
   tier: tierEnum("tier").notNull().default("free"),
+  stripeCustomerId: text("stripe_customer_id").unique(),
   unitPref: unitPrefEnum("unit_pref").notNull().default("metric"),
   locale: text("locale").notNull().default("en"),
   createdAt: timestamp("created_at").notNull().defaultNow(),