♻️(backend) generalize YProvider API config

Abstracted base URL and API key under 'y-provider' for
reuse in future endpoints, aligning with microservice naming.

Please note the YProvider API here is internal to the cluster.
In facts, we don't want these endpoints to be exposed by any ingress

src/backend/core/services/converter_services.py

@@ -31,7 +31,7 @@ class YdocConverter:
     @property
     def auth_header(self):
         """Build microservice authentication header."""
-        return settings.CONVERSION_API_KEY
+        return settings.Y_PROVIDER_API_KEY
 
     def convert_markdown(self, text):
         """Convert a Markdown text into our internal format using an external microservice."""
@@ -41,7 +41,7 @@ class YdocConverter:
 
         try:
             response = requests.post(
-                settings.CONVERSION_API_URL,
+                f"{settings.Y_PROVIDER_API_BASE_URL}{settings.CONVERSION_API_ENDPOINT}/",
                 json={
                     "content": text,
                 },