fix(deepgram): revert to /v1/auth/grant for temporary JWT tokens

- /v1/projects/{id}/keys creates permanent API keys, not WebSocket-compatible JWT tokens
- /v1/auth/grant requires Member-scoped API key (now configured)
- Remove DEEPGRAM_PROJECT_ID dependency
- Update tests

Typecheck: OK · Tests: 241/241 ✅

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -6,7 +6,6 @@ SUPABASE_SERVICE_ROLE_KEY=xxx
 DEEPSEEK_API_KEY=xxx
 GEMINI_API_KEY=xxx
 DEEPGRAM_API_KEY=xxx
-DEEPGRAM_PROJECT_ID=xxx
 
 # Stripe
 STRIPE_SECRET_KEY=xxx