feat(pitch-deck): English email templates, investor language preference, link-only invite mode

- Add English email template variants (greeting, message, closing, subject, CTA copy)
- Add `preferred_lang` column to `pitch_investors` — stored per investor, deck opens in that language by default
- Invite form: DE/EN language toggle that switches email defaults and pitch language setting
- Invite form: "Send email" toggle — when off, creates investor + returns magic link without sending email (for cold outreach attachment)
- `app/page.tsx`: initializes pitch language from investor's `preferred_lang` before first render (no flash)
- Migration 007 added to `/api/admin/migrate` route for production rollout

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

pitch-deck/app/api/admin/migrate/route.ts

@@ -160,6 +160,8 @@ export async function POST(request: NextRequest) {
     `ALTER TABLE dataroom_documents ADD COLUMN IF NOT EXISTS description_en TEXT`,
     `ALTER TABLE dataroom_investor_uploads ADD COLUMN IF NOT EXISTS description_de TEXT`,
     `ALTER TABLE dataroom_investor_uploads ADD COLUMN IF NOT EXISTS description_en TEXT`,
+    // 007 — investor preferred language
+    `ALTER TABLE pitch_investors ADD COLUMN IF NOT EXISTS preferred_lang VARCHAR(5) NOT NULL DEFAULT 'de'`,
   ]
 
   for (const sql of statements) {