fix(docker): add SUPABASE_INTERNAL_URL for server-side Supabase access

Server-side code (proxy.ts, SSR, API routes) now uses SUPABASE_INTERNAL_URL
(http://supabase-kong:8000) instead of the external domain. This avoids
hairpin NAT / DNS resolution issues where Docker containers can't reach
their own external domain through the reverse proxy.

Browser-side JS still uses the external URL (baked at build time).

Dockerfile

@@ -4,7 +4,7 @@ WORKDIR /app
 
 # --- Install + Build in one stage ---
 FROM base AS builder
-ARG CACHE_BUST=5
+ARG CACHE_BUST=6
 COPY . .
 RUN pnpm install --no-frozen-lockfile
 ENV NEXT_TELEMETRY_DISABLED=1