feat: wire Supabase credentials and expose tables via Data API

Add supabase/config.sh with project URL and publishable key (safe to
commit — RLS restricts to INSERT only). Update telemetry-sync,
community-dashboard, and update-check to source the config and
include proper auth headers for the Supabase REST API.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

supabase/config.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+# Supabase project config for gstack telemetry
+# These are PUBLIC keys — safe to commit (like Firebase public config).
+# RLS policies restrict what the anon/publishable key can do (INSERT only).
+
+GSTACK_SUPABASE_URL="https://frugpmstpnojnhfyimgv.supabase.co"
+GSTACK_SUPABASE_ANON_KEY="sb_publishable_tR4i6cyMIrYTE3s6OyHGHw_ppx2p6WK"
+
+# Telemetry ingest endpoint (Data API)
+GSTACK_TELEMETRY_ENDPOINT="${GSTACK_SUPABASE_URL}/rest/v1"