feat(pitch-deck): change preferred_lang for existing investors from detail page

- GET /api/admin/investors/:id now returns preferred_lang
- PATCH /api/admin/investors/:id accepts preferred_lang (de/en), validates value
- Investor detail page: DE/EN toggle in the Pitch Version card, instant save on click

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

pitch-deck/app/api/admin/investors/[id]/route.ts

@@ -19,7 +19,7 @@ export async function GET(request: NextRequest, ctx: RouteContext) {
     pool.query(
       `SELECT i.id, i.email, i.name, i.company, i.status, i.last_login_at, i.login_count,
               i.created_at, i.updated_at, i.first_activity_at, i.data_masked_at,
-              i.assigned_version_id, i.is_showcase,
+              i.assigned_version_id, i.is_showcase, i.preferred_lang,
               v.name AS version_name, v.status AS version_status
        FROM pitch_investors i
        LEFT JOIN pitch_versions v ON v.id = i.assigned_version_id
@@ -68,14 +68,14 @@ export async function PATCH(request: NextRequest, ctx: RouteContext) {
 
   const { id } = await ctx.params
   const body = await request.json().catch(() => ({}))
-  const { name, company, assigned_version_id, is_showcase } = body
+  const { name, company, assigned_version_id, is_showcase, preferred_lang } = body
 
-  if (name === undefined && company === undefined && assigned_version_id === undefined && is_showcase === undefined) {
-    return NextResponse.json({ error: 'name, company, assigned_version_id, or is_showcase required' }, { status: 400 })
+  if (name === undefined && company === undefined && assigned_version_id === undefined && is_showcase === undefined && preferred_lang === undefined) {
+    return NextResponse.json({ error: 'name, company, assigned_version_id, is_showcase, or preferred_lang required' }, { status: 400 })
   }
 
   const before = await pool.query(
-    `SELECT name, company, assigned_version_id, is_showcase FROM pitch_investors WHERE id = $1`,
+    `SELECT name, company, assigned_version_id, is_showcase, preferred_lang FROM pitch_investors WHERE id = $1`,
     [id],
   )
   if (before.rows.length === 0) {
@@ -98,8 +98,8 @@ export async function PATCH(request: NextRequest, ctx: RouteContext) {
 
   // Use null to clear version assignment, undefined to leave unchanged
   const versionValue = assigned_version_id === undefined ? before.rows[0].assigned_version_id : (assigned_version_id || null)
-
   const showcaseValue = is_showcase !== undefined ? Boolean(is_showcase) : before.rows[0].is_showcase
+  const langValue = preferred_lang === 'en' || preferred_lang === 'de' ? preferred_lang : before.rows[0].preferred_lang
 
   const { rows } = await pool.query(
     `UPDATE pitch_investors SET
@@ -107,10 +107,11 @@ export async function PATCH(request: NextRequest, ctx: RouteContext) {
        company = COALESCE($2, company),
        assigned_version_id = $4,
        is_showcase = $5,
+       preferred_lang = $6,
        updated_at = NOW()
      WHERE id = $3
-     RETURNING id, email, name, company, status, assigned_version_id, is_showcase`,
-    [name ?? null, company ?? null, id, versionValue, showcaseValue],
+     RETURNING id, email, name, company, status, assigned_version_id, is_showcase, preferred_lang`,
+    [name ?? null, company ?? null, id, versionValue, showcaseValue, langValue],
   )
 
   const action = assigned_version_id !== undefined && assigned_version_id !== before.rows[0].assigned_version_id