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/recipes.ts

@@ -49,6 +49,7 @@ export const recipes = pgTable("recipes", {
 }, (t) => [
   index("recipes_author_idx").on(t.authorId),
   index("recipes_visibility_idx").on(t.visibility),
+  index("recipes_dietary_tags_gin").using("gin", t.dietaryTags),
 ]);
 
 export const ingredients = pgTable("ingredients", {