fix: serve a leaf certificate, not the CA

LibreChat served the lab CA's own certificate as its server certificate — a
CA:TRUE certificate valid for the whole *.sttlab.local wildcard, from a
hand-made tls.yaml that also carried the CA's private key. Gitignored, so never
leaked, but the only service in the estate still copying the CA around instead
of being issued a leaf.

Now a cert-manager Certificate for librechat.sttlab.local: CA:FALSE, one
dnsNames, key generated in the cluster and rotated there.

The two uses of that Secret had to be separated first, which is the part worth
knowing: sttlab-tls was both the ingress certificate and the CA bundle mounted
as NODE_EXTRA_CA_CERTS for outbound calls. Replacing it with a leaf alone would
have left LibreChat unable to verify Keycloak. The CA certificate now comes from
ca-configmap.yaml — the certificate, never the key, committed like the agents'.

Verified: the ingress serves CA:FALSE with SAN librechat.sttlab.local, the pod's
outbound call to Keycloak returns 200 with TLS verified, and the old secret is
deleted.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,8 @@
 # Secrets — ne jamais committer de valeurs réelles
 secret.yaml
-tls.yaml
+# tls.yaml : plus de fichier TLS local. Le certificat de service est émis par
+# cert-manager (ingress/certificate.yaml) et sa clé reste dans le cluster ; le
+# certificat du CA, lui, est public et versionné dans ca-configmap.yaml.
 
 # Certificats TLS
 *.crt