feat(selfhost): complete Supabase removal migration

Phase 1 — Pattern library
- Add src/lib/api.ts (typed PostgREST client, anon-key)
- Add src/lib/svc-fetch.ts (service-role fetch + PostgREST client)
- Add src/lib/db-types.ts (Database generic, RowOf helper)
- Add src/lib/auth-admin.ts (better-auth admin wrappers: createUser, listUsers, removeUser, requestPasswordReset, validateSession)

Phase 2 — Server action migration
- Migrate all 67 server actions off @supabase/supabase-js to svcApi/svcRpc
- Replace service.auth.admin.* with authAdmin.* (better-auth)
- Replace publicApi.auth.* with authAdmin.* (better-auth)
- Add typed single() null guards + nullable column fallbacks

Phase 3 — Delete mock data + debug routes
- Remove if(useMockData) branches from 7 files (-226 lines)
- Delete src/lib/mock-data.ts (no longer referenced)
- Delete 7 debug API routes (debug-admin-users, debug-auth, debug-cookie, debug-env, debug-get-admin-users, debug-hello, debug-me)

Phase 4 — Hard cut
- Delete src/lib/supabase.ts (orphan — no importers)
- Delete src/app/api/supabase + src/app/api/supabase-test routes
- Remove @supabase/ssr + @supabase/supabase-js from package.json
- Rename env vars: NEXT_PUBLIC_SUPABASE_URL → NEXT_PUBLIC_API_URL, NEXT_PUBLIC_SUPABASE_ANON_KEY → NEXT_PUBLIC_API_ANON_KEY, SUPABASE_SERVICE_ROLE_KEY → POSTGREST_SERVICE_KEY

Phase 5 — Verify
- npx tsc --noEmit: 0 errors
- npm run lint: 0 new errors from migration (104 pre-existing errors unrelated)
- npm run build: same failure mode as main worktree (PostgREST-dependent prerender), no regression

Net: 158 files changed, +1640 / -1903 lines (-263 net)

src/app/admin/page.tsx

@@ -1,5 +1,5 @@
 import Link from "next/link";
-import { supabase } from "@/lib/supabase";
+import { api } from "@/lib/api";
 import { getAdminUser } from "@/lib/admin-permissions";
 import { isFeatureEnabled } from "@/lib/feature-flags";
 import { getBillingOverview } from "@/actions/billing/billing-overview";
@@ -30,7 +30,7 @@ export default async function AdminPage() {
   // (0/0/0) usage values and contradicts the billing page's "Products 1/25".
   let dashboardBrandId: string | null = adminUser?.brand_id ?? null;
   if (!dashboardBrandId && adminUser?.role === "platform_admin") {
-    const { data: firstBrand } = await supabase
+    const { data: firstBrand } = await api
       .from("brands")
       .select("id")
       .limit(1)