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/api/v1/ai-keys/route.ts

@@ -1,7 +1,7 @@
 import { NextResponse } from "next/server";
 import { z } from "zod";
 import { db, userAiKeys, eq, and } from "@epicure/db";
-import { requireDeveloper } from "@/lib/api-auth";
+import { requireByok } from "@/lib/api-auth";
 import { encrypt } from "@/lib/encrypt";
 import { applyRateLimit } from "@/lib/rate-limit";
 
@@ -13,7 +13,7 @@ const PostSchema = z.object({
 });
 
 export async function GET() {
-  const { session, response } = await requireDeveloper();
+  const { session, response } = await requireByok();
   if (response) return response;
 
   const keys = await db.query.userAiKeys.findMany({
@@ -25,7 +25,7 @@ export async function GET() {
 }
 
 export async function POST(req: Request) {
-  const { session, response } = await requireDeveloper();
+  const { session, response } = await requireByok();
   if (response) return response;
 
   const limited = await applyRateLimit(`rl:ai-keys:${session!.user.id}`, 5, 3600);