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/actions/admin/audit.ts

@@ -1,15 +1,6 @@
 "use server";
 
-import { createClient as createServiceClient } from "@supabase/supabase-js";
-
-function getServiceClient() {
-  const roleKey = process.env.SUPABASE_SERVICE_ROLE_KEY;
-  if (!roleKey) throw new Error("SUPABASE_SERVICE_ROLE_KEY is not set");
-  return createServiceClient(
-    process.env.NEXT_PUBLIC_SUPABASE_URL!,
-    roleKey,
-  );
-}
+import { svcRpc } from "@/lib/svc-fetch";
 
 type AdminActionPayload = {
   action_type: "create" | "update" | "delete";
@@ -30,8 +21,7 @@ type UserActivityPayload = {
 
 export async function logAdminAction(payload: AdminActionPayload): Promise<void> {
   try {
-    const service = getServiceClient();
-    await service.rpc("log_admin_action", {
+    await svcRpc("log_admin_action", {
       p_payload: {
         action_type: payload.action_type,
         admin_id: payload.admin_id ?? null,
@@ -48,8 +38,7 @@ export async function logAdminAction(payload: AdminActionPayload): Promise<void>
 
 export async function logUserActivity(payload: UserActivityPayload): Promise<void> {
   try {
-    const service = getServiceClient();
-    await service.rpc("log_user_activity", {
+    await svcRpc("log_user_activity", {
       p_payload: {
         user_id: payload.user_id,
         activity_type: payload.activity_type,