Checkpoint: v4.0 media service, compose deploy, and verified docs

.env.example

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+NODE_ENV=production
+PORT=3000
+
+# App auth / storage / database
+DATABASE_URL=mysql://user:password@127.0.0.1:4000/tennis_training_hub
+JWT_SECRET=replace-with-strong-secret
+VITE_APP_ID=tennis-training-hub
+OAUTH_SERVER_URL=
+OWNER_OPEN_ID=
+BUILT_IN_FORGE_API_URL=
+BUILT_IN_FORGE_API_KEY=
+VITE_OAUTH_PORTAL_URL=
+VITE_FRONTEND_FORGE_API_URL=
+VITE_FRONTEND_FORGE_API_KEY=
+
+# Optional direct media URL override for browser builds
+VITE_MEDIA_BASE_URL=/media
+
+# Local app-to-media proxy for development or direct container access
+MEDIA_SERVICE_URL=http://127.0.0.1:8081