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/messages/en.json

@@ -420,7 +420,9 @@
     "nutrition": "Nutrition",
     "apiKeys": "API Keys",
     "webhooks": "Webhooks",
-    "developerLockedNotice": "Developer access required. Ask an admin to enable it for your account.",
+    "developerLockedNotice": "Developer access required. Ask an admin to enable it for your account, or upgrade to a paid plan to enable it yourself.",
+    "developerSelfServeNotice": "Enable developer access to use webhooks and API keys — included free with your plan, no extra charge.",
+    "byokLockedNotice": "BYOK access required. Ask an admin to enable it for your account.",
     "usage": {
       "title": "Usage & limits",
       "description": "AI calls reset monthly ({month}). Recipes and storage are lifetime totals.",