M0: DB schema, RLS policies, JWT claims hook, seed, Supabase clients

Adds db/schema.sql (12 tables), db/policies.sql (RLS on all 12,
audit_log append-only), db/auth-hook.sql (role/org_id into JWT per
AD-2), db/seed.sql (org + 3 departments, part 2 deferred to M1 auth).
Wires lib/supabase/{client,server,service}.ts per AD-3 and adds
/db-check page confirming DB connectivity and RLS deny-by-default.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FcktbLXSSXzx23GCue813e

src/lib/supabase/client.ts

@@ -0,0 +1,8 @@
+import { createBrowserClient } from "@supabase/ssr";
+
+export function createClient() {
+  return createBrowserClient(
+    process.env.NEXT_PUBLIC_SUPABASE_URL!,
+    process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!
+  );
+}