feat: switch embeddings from Voyage AI to Google Gemini text-embedding-004

- embedText: call Gemini REST API (768-dim) instead of Voyage (1024-dim)
- Migration: drop+recreate incidents.embedding as vector(768), update
  match_incidents function, swap VOYAGE_API_KEY → GOOGLE_AI_API_KEY in app_settings
- Settings UI: relabel to "Google AI API Key (Embeddings)"
- All call sites updated (incidents POST, similar GET)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CPf5Rc8QPx6V8KLEEgfKEQ

app/api/settings/route.ts

@@ -6,7 +6,7 @@ import { createClient } from '@/lib/supabase/server'
 const ALLOWED_KEYS = [
   'ANTHROPIC_API_KEY',
   'DEEPSEEK_API_KEY',
-  'VOYAGE_API_KEY',
+  'GOOGLE_AI_API_KEY',
   'META_WHATSAPP_PHONE_NUMBER_ID',
   'META_WHATSAPP_ACCESS_TOKEN',
 ] as const