Add an AI chat in 3 steps

// Streaming chat proxy — keeps the model key server-side. The app POSTs
// {messages} with the signed-in user's JWT; we stream back `data: {"delta":"…"}`.
const OPENAI_KEY = Deno.env.get("OPENAI_API_KEY")!
const MODEL = Deno.env.get("CHAT_MODEL") ?? "gpt-4o-mini"

Deno.serve(async (req) => {
  if (req.method !== "POST") return new Response("Method not allowed", { status: 405 })
  // Supabase forwards the caller's JWT; require it so only signed-in users spend tokens.
  if (!req.headers.get("Authorization")) return new Response("Unauthorized", { status: 401 })

  const { messages } = await req.json()
  if (!Array.isArray(messages)) return new Response("Bad request", { status: 400 })

  const upstream = await fetch("https://api.openai.com/v1/chat/completions", {
    method: "POST",
    headers: { "Content-Type": "application/json", Authorization: `Bearer ${OPENAI_KEY}` },
    body: JSON.stringify({ model: MODEL, messages, stream: true }),
  })
  if (!upstream.ok || !upstream.body) return new Response(`Upstream error (${upstream.status})`, { status: 502 })

  // Re-emit the upstream SSE as our minimal {delta} contract.
  const stream = new ReadableStream({
    async start(controller) {
      const enc = new TextEncoder(), dec = new TextDecoder()
      const reader = upstream.body!.getReader()
      let buffer = ""
      for (;;) {
        const { value, done } = await reader.read()
        if (done) break
        buffer += dec.decode(value, { stream: true })
        const lines = buffer.split("\n")
        buffer = lines.pop() ?? ""
        for (const line of lines) {
          if (!line.startsWith("data:")) continue
          const payload = line.slice(5).trim()
          if (payload === "[DONE]") { controller.enqueue(enc.encode("data: [DONE]\n\n")); continue }
          try {
            const delta = JSON.parse(payload).choices?.[0]?.delta?.content
            if (delta) controller.enqueue(enc.encode(`data: ${JSON.stringify({ delta })}\n\n`))
          } catch { /* keep-alive / partial line */ }
        }
      }
      controller.close()
    },
  })
  return new Response(stream, {
    headers: { "Content-Type": "text/event-stream", "Cache-Control": "no-cache", Connection: "keep-alive" },
  })
})