fix(web): proxy client API calls through Next.js to fix production connectivity

NEXT_PUBLIC_API_URL was undefined at build time, causing client bundles to
fall back to http://localhost:3001 — unreachable from the browser in prod.

- Add /api-proxy rewrite in next.config.ts (forwards to API_INTERNAL_URL at runtime)
- Bake NEXT_PUBLIC_API_URL=/api-proxy at build time in Dockerfile
- Fix api.ts to prefer API_INTERNAL_URL for server-side calls
- Fix docker-compose.prod.yml: set NEXT_PUBLIC_API_URL=http://api:3001 for runtime server-side code

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/Dockerfile

@@ -14,6 +14,7 @@ COPY --from=deps /app/ ./
 COPY apps/web/ ./apps/web/
 COPY packages/shared/ ./packages/shared/
 COPY tsconfig.base.json ./
+ENV NEXT_PUBLIC_API_URL=/api-proxy
 RUN pnpm --filter=@tessera/web build
 
 FROM node:24-alpine AS runner