fix: merge env AI_MODELS_CONFIG with admin panel providers

Previously, saving in the admin panel wrote a complete AI_MODELS_CONFIG
into settings.json, which (by overlay precedence) replaced any config
from .env or ai-models.json — admins lost their env-configured models.

The panel no longer writes AI_MODELS_CONFIG. Instead its providers are
merged with the env baseline at read time in loadRawServerModelsConfig,
and panel credentials go to ADMIN_-prefixed env vars wired up via
apiKeyEnv/baseUrlEnv so they never shadow standard vars. Env-based
providers now appear read-only in the panel, name clashes are rejected,
and a panel default overrides the env default. data/ is now gitignored.

.gitignore

@@ -76,3 +76,6 @@ ai-models.json
 # local backups
 *.bak
 .gstack/
+
+# admin panel settings (contains secrets)
+data/