Improve AI music prompts and audio metadata

.env.example

@@ -96,9 +96,16 @@ EMAIL_SMTP_USER=
 EMAIL_SMTP_PASS=
 EMAIL_FROM_NAME=Oudelaa
 EMAIL_FROM_EMAIL=
+# Enables POST /api/v1/media/ai/text-to-music.
+# Uses Google Vertex AI publisher model endpoint, not GEMINI_API_KEY directly.
+# Auth priority:
+# 1. AI_MUSIC_API_KEY when set.
+# 2. GOOGLE_APPLICATION_CREDENTIALS_JSON_BASE64 when set.
+# 3. GOOGLE_APPLICATION_CREDENTIALS file path handled by google-auth-library.
 AI_MUSIC_ENABLED=false
 AI_MUSIC_API_KEY=
-AI_MUSIC_PROJECT_ID=
+GOOGLE_APPLICATION_CREDENTIALS_JSON_BASE64=
+AI_MUSIC_PROJECT_ID=accordev
 AI_MUSIC_LOCATION=us-central1
 AI_MUSIC_MODEL=lyria-002