fix: no user ever had a username, so people search found nobody

Root cause of "still cannot find users": username was optional in
better-auth's schema and no signup form or settings page ever set it,
but search/profile/follow all key on username, not user id. Every
account now gets a unique one auto-generated (from name/email) in the
user.create.before hook — covers email/password and OAuth signups.
Added a one-off db:backfill-usernames script for accounts created
before this fix and ran it against the current database.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "epicure",
-  "version": "0.13.0",
+  "version": "0.13.1",
   "private": true,
   "scripts": {
     "dev": "pnpm --filter web dev",
@@ -10,6 +10,7 @@
     "db:generate": "pnpm --filter @epicure/db generate",
     "db:migrate": "pnpm --filter @epicure/db migrate",
     "db:seed": "pnpm --filter @epicure/db seed",
+    "db:backfill-usernames": "pnpm --filter @epicure/db backfill-usernames",
     "db:studio": "pnpm --filter @epicure/db studio"
   },
   "devDependencies": {