fix(config): add default empty values for optional env vars

Declare empty-string defaults for all optional env vars as Symfony
parameters so the container compiles without errors when GITHUB_USER,
GITLAB_TOKEN, GITEA_URL, etc. are not set in the environment.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/services.yaml

@@ -1,3 +1,15 @@
+parameters:
+    env(APP_SECRET): ''
+    env(ALLOWED_HOSTS): ''
+    env(GITHUB_USER): ''
+    env(GITHUB_TOKEN): ''
+    env(GITLAB_USER): ''
+    env(GITLAB_TOKEN): ''
+    env(GITLAB_URL): ''
+    env(GITEA_USER): ''
+    env(GITEA_TOKEN): ''
+    env(GITEA_URL): ''
+
 services:
     _defaults:
         autowire: true