build(image): ship the ldap extra so LDAP sign-in works in the image (#5036)

The Settings -> Auth LDAP block had no ldap3 to run on: every LDAP login
logged "ldap3 not installed" and failed as "Invalid credentials".
Operator chose shipping it over hiding the option.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -19,10 +19,11 @@ WORKDIR /app
 
 COPY pyproject.toml .
 COPY steward/ steward/
-# Bundle the extras whose first-party plugins ship in the image: [ansible] for
-# the playbook runner, [snmp] (pysnmp) for the SNMP poller. Without them those
-# bundled plugins load but silently no-op for want of their runtime dependency.
-RUN pip install --no-cache-dir '.[ansible,snmp]'
+# Bundle the extras whose features ship in the image: [ansible] for the
+# playbook runner, [snmp] (pysnmp) for the SNMP poller, [ldap] (ldap3, pure
+# Python) for LDAP sign-in under Settings -> Auth. Without them those features
+# load but fail for want of their runtime dependency.
+RUN pip install --no-cache-dir '.[ansible,snmp,ldap]'
 
 COPY alembic.ini .
 # First-party plugins ship inside the image (bundled root at /app/plugins).