Improve CORS config

[doringeman]

Allow passing a list of allowed origins or retrieve them from DMR_ORIGINS.

You can test this with https://github.com/ilopezluna/chat2cart/tree/streaming and the following patch:

diff --git a/web/static/js/chat.js b/web/static/js/chat.js
index 1ecb6f0..0c6ccec 100644
--- a/web/static/js/chat.js
+++ b/web/static/js/chat.js
@@ -637,11 +637,11 @@ async function updateModelSelector(provider) {
             let endpoint;
             if (provider === 'dmr') {
                 // Use backend endpoint for DMR models
-                endpoint = '/api/v1/models/dmr';
+                endpoint = 'http://localhost:12434/engines/v1/models';
                 const response = await fetch(endpoint);
                 if (response.ok) {
                     const data = await response.json();
-                    models = data.models.map(model => ({
+                    models = data.data.map(model => ({
                         id: model.id,
                         name: model.id // Use the model ID as the display name
                     }));

• Disable CORS by not setting DMR_ORIGINS with MODEL_RUNNER_PORT=12434 make run.
• Enable CORS for all with DMR_ORIGINS=* MODEL_RUNNER_PORT=12434 make run.
• Enable it for a specific origin with DMR_ORIGINS=http://localhost:3131 MODEL_RUNNER_PORT=12434 make run (run chat2cart on port 3131).
  By default, it will restrict all.