feat: self-serve developer access for paid tiers, split BYOK into its own permission (v0.72.0)

Splits what was one isDeveloper flag (gating webhooks, API keys, AND
BYOK together) into two independent permissions:

- isDeveloper (webhooks + self-serve API keys): admin-toggled as
  before, but now ALSO self-serve -- PATCH /api/v1/users/me/developer-access
  lets any paid-tier (tier !== "free") user turn it on themselves, no
  added fee. Free tier still needs an admin grant. Turning it off is
  always self-serve regardless of tier, since revoking your own
  access needs no gatekeeping. canSelfServeDeveloperAccess() in
  lib/permissions.ts is the single check for "is this tier eligible."

- isByokEnabled (BYOK AI provider keys): new column, admin-only, no
  self-serve path at all -- routing real AI provider spend through
  Epicure on the user's own key warrants a manual admin check-in that
  webhooks/API access don't need. requireByok() replaces
  requireDeveloper() on the three ai-keys routes.

Migration adds is_byok_enabled and grandfathers in anyone who already
has a BYOK key configured (the earlier grandfather migration only
covered the combined isDeveloper flag, which BYOK no longer reads).

Settings UI: webhooks/API-keys pages show a self-serve "Enable"
toggle for paid-tier non-developers instead of the locked notice
(still shown to free-tier users), plus a "Disable developer access"
link once enabled. Settings -> AI's BYOK section now checks
isByokEnabled instead of isDeveloper -- unaffected by the self-serve
change, still fully admin-gated.

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

apps/web/app/(app)/settings/ai/page.tsx

@@ -28,7 +28,7 @@ export default async function AiSettingsPage() {
     }),
     // Tier comes from the DB, not the (up to 5-minute-stale) session cookie
     // cache, so a just-changed tier's limits show up immediately here.
-    db.query.users.findFirst({ where: eq(users.id, session.user.id), columns: { tier: true, isDeveloper: true } }),
+    db.query.users.findFirst({ where: eq(users.id, session.user.id), columns: { tier: true, isByokEnabled: true } }),
   ]);
 
   const [tierDef, usage, recipeCount, storageUsedMb] = await Promise.all([
@@ -81,10 +81,10 @@ export default async function AiSettingsPage() {
             {m.settings.byok.description}
           </p>
         </div>
-        {dbUser?.isDeveloper ? (
+        {dbUser?.isByokEnabled ? (
           <ByokManager initialKeys={aiKeys.map((k) => k.provider)} />
         ) : (
-          <DeveloperLockedNotice message={m.settings.developerLockedNotice} />
+          <DeveloperLockedNotice message={m.settings.byokLockedNotice} />
         )}
       </section>