feat: PC7 chatbot V1 onglet centre HAUT + endpoint Astro proxy SSR

- ChatbotV2.vue : Vue island, thread chat (input + messages bot/user),
  persistance sessionStorage, bandeau beta '120 fiches AEP, RAG-PE bientot',
  gestion erreurs 429/502/504 ; pas de streaming ni markdown V1
- /api/chatbot.ts : endpoint Astro server proxy POST vers CHATBOT_UPSTREAM
  (default https://aep.trans-former.fr/api/chatbot), timeout 25s,
  body { question, history } -> upstream classique chatbot AEP Mistral Small
- astro.config.mjs : output 'server' + adapter @astrojs/node standalone
  (Astro 6 a supprime mode hybrid ; on opt-in prerender sur les pages)
- Toutes les pages publiques (index, manifeste, manifeste/commander,
  a-propos, mentions-legales) ont 'export const prerender = true'
- ColCentre.astro : remplace ChatbotPlaceholder par ChatbotV2 dans le tab
- .env.example : ajoute CHATBOT_UPSTREAM (V1.5 = switch LightRAG-PE 1 ligne)

Decision V1 : endpoint AEP /api/chatbot (classique, repond bien) au lieu
de /api/chatbot-v2 qui retourne v2_ready=false ('base vectorielle en cours').
Bandeau beta reste valide ; switch v2 quand ready cote AEP via env var.

Note PC8 deploy : Coolify doit booter avec 'node ./dist/server/entry.mjs'
(SSR Node standalone) au lieu de servir dist/client/ static.

Test end-to-end OK : SSR boot port 4399 + curl POST /api/chatbot ->
reponse_texte 800+ chars de l'AEP backend.

src/pages/api/chatbot.ts

@@ -0,0 +1,65 @@
+// PC7 — Endpoint proxy POST /api/chatbot
+// Forward la question vers CHATBOT_UPSTREAM (default : aep.trans-former.fr/api/chatbot).
+// Tunnel CORS (l'upstream ne sert pas de header CORS pour trans-former.fr) + timeout 25s.
+
+import type { APIRoute } from 'astro'
+
+export const prerender = false
+
+const UPSTREAM =
+  import.meta.env.CHATBOT_UPSTREAM || 'https://aep.trans-former.fr/api/chatbot'
+const TIMEOUT_MS = 25_000
+
+const jsonResponse = (status: number, payload: unknown) =>
+  new Response(JSON.stringify(payload), {
+    status,
+    headers: { 'Content-Type': 'application/json; charset=utf-8' },
+  })
+
+export const POST: APIRoute = async ({ request }) => {
+  let body: { question?: string; q?: string; history?: unknown }
+
+  try {
+    body = await request.json()
+  } catch {
+    return jsonResponse(400, { error: 'invalid_json' })
+  }
+
+  const question = (body.question || body.q || '').toString().trim()
+  if (!question) {
+    return jsonResponse(400, { error: 'missing_question' })
+  }
+
+  const ctrl = new AbortController()
+  const timer = setTimeout(() => ctrl.abort(), TIMEOUT_MS)
+
+  try {
+    const upstream = await fetch(UPSTREAM, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({
+        question,
+        history: Array.isArray(body.history) ? body.history : [],
+      }),
+      signal: ctrl.signal,
+    })
+    clearTimeout(timer)
+
+    const text = await upstream.text()
+    const contentType =
+      upstream.headers.get('content-type') || 'application/json; charset=utf-8'
+
+    return new Response(text, {
+      status: upstream.status,
+      headers: { 'Content-Type': contentType },
+    })
+  } catch (e) {
+    clearTimeout(timer)
+    const err = e as Error
+    const aborted = err.name === 'AbortError'
+    return jsonResponse(aborted ? 504 : 502, {
+      error: aborted ? 'upstream_timeout' : 'upstream_failed',
+      detail: err.message || 'unknown',
+    })
+  }
+}