Rename OIDC strategy from :rauthy to :oidc, update callback path

- Rename AshAuthentication strategy from :oidc :rauthy to :oidc :oidc;
  generated actions are now register_with_oidc / sign_in_with_oidc.
- Update config keys (:rauthy → :oidc) in dev.exs and runtime.exs.
- Update default_redirect_uri to /auth/user/oidc/callback everywhere.
- Rename Mv.Accounts helper functions accordingly.
- Update Mv.Secrets, AuthController, link_oidc_account_live and all tests.
- Update docker-compose.prod.yml, .env.example, README and docs.

IMPORTANT: OIDC providers must be updated to use the new redirect URI
/auth/user/oidc/callback instead of /auth/user/rauthy/callback.

README.md

@@ -142,7 +142,7 @@ Mila uses OIDC for Single Sign-On. In development, a local **Rauthy** instance i
 3. Login with "admin@localhost" and password from `BOOTSTRAP_ADMIN_PASSWORD_PLAIN` in docker-compose.yml
 4. add client from the admin panel
     - Client ID: mv
-    - redirect uris: http://localhost:4000/auth/user/rauthy/callback
+    - redirect uris: http://localhost:4000/auth/user/oidc/callback
     - Authorization Flows: authorization_code
     - allowed origins: http://localhost:4000
     - access/id token algortihm: RS256 (EDDSA did not work for me, found just few infos in the ashauthentication docs)
@@ -155,11 +155,11 @@ Now you can log in to Mila via OIDC!
 
 Mila works with any OIDC-compliant provider. The internal strategy is named `:rauthy`, but this is just a name — it works with any provider.
 
-**Important:** The redirect URI must always end with `/auth/user/rauthy/callback`.
+**Important:** The redirect URI must always end with `/auth/user/oidc/callback`.
 
 Example for Authentik:
 1. Create an OAuth2/OpenID Provider in Authentik
-2. Set the redirect URI to: `https://your-domain.com/auth/user/rauthy/callback`
+2. Set the redirect URI to: `https://your-domain.com/auth/user/oidc/callback`
 3. Configure environment variables:
    ```bash
    DOMAIN=your-domain.com                    # or PHX_HOST=your-domain.com
@@ -168,7 +168,7 @@ Example for Authentik:
    OIDC_CLIENT_SECRET=your-client-secret     # or use OIDC_CLIENT_SECRET_FILE
    ```
 
-The `OIDC_REDIRECT_URI` is auto-generated as `https://{DOMAIN}/auth/user/rauthy/callback` if not explicitly set.  
+The `OIDC_REDIRECT_URI` is auto-generated as `https://{DOMAIN}/auth/user/oidc/callback` if not explicitly set.  
 
 ## ⚙️ Configuration
 
@@ -238,7 +238,7 @@ For testing the production Docker build locally:
    # OIDC_CLIENT_ID=mv
    # OIDC_BASE_URL=http://localhost:8080/auth/v1
    # OIDC_CLIENT_SECRET=<from-your-oidc-provider>
-   # OIDC_REDIRECT_URI is auto-generated as https://{DOMAIN}/auth/user/rauthy/callback
+   # OIDC_REDIRECT_URI is auto-generated as https://{DOMAIN}/auth/user/oidc/callback
    
    # Alternative: Use _FILE variables for Docker secrets (takes priority over regular vars):
    # SECRET_KEY_BASE_FILE=/run/secrets/secret_key_base