docs: document the BAMBUDDY_OIDC_* variables in .env.example

Follows the BAMBUDDY_LOCAL_LOGIN block's style: what it is for, when it
activates, and the two things an operator cannot guess from the variable names
-- that removing the config disables rather than deletes the provider, because
deleting would permanently drop every account link, and that the UI shows it
read-only because startup would overwrite an edit anyway.

Also states why auto-link is refused without verified emails, since that is
the one setting that will be silently skipped if someone gets it wrong.

Refs #2593

.env.example

@@ -66,3 +66,43 @@ LOG_TO_FILE=true
 # LDAP is governed by its own ldap_enabled toggle and is not affected.
 # Leave unset for normal operation.
 # BAMBUDDY_LOCAL_LOGIN=true
+
+# --- OIDC provider from the environment (#2593) ------------------------------
+# Defines ONE OIDC provider declaratively, for deployments that are managed by
+# compose files or GitOps and never touch the settings UI. Providers created in
+# the UI are unaffected and keep working alongside this one.
+#
+# Activates only when all four required vars below are set; an empty value
+# counts as unset. The provider is written on startup and re-applied on every
+# boot, so the UI shows it as read-only and the API refuses to change it -- an
+# edit there would be reverted at the next restart anyway.
+#
+# Removing the vars DISABLES the provider rather than deleting it: accounts
+# linked to it would otherwise lose their link permanently. Re-adding the vars
+# enables it again with those links intact.
+#
+# If you lock yourself out, BAMBUDDY_LOCAL_LOGIN=true above is the way back in.
+#
+# Required:
+# BAMBUDDY_OIDC_NAME=Keycloak
+# BAMBUDDY_OIDC_ISSUER_URL=https://sso.example.com/realms/main
+# BAMBUDDY_OIDC_CLIENT_ID=bambuddy
+# BAMBUDDY_OIDC_CLIENT_SECRET=your-client-secret
+#
+# Optional, shown with their defaults:
+# BAMBUDDY_OIDC_SCOPES=openid email profile
+# BAMBUDDY_OIDC_ENABLED=true
+# BAMBUDDY_OIDC_AUTO_CREATE_USERS=false
+# BAMBUDDY_OIDC_AUTO_LINK_EXISTING=false
+# BAMBUDDY_OIDC_EMAIL_CLAIM=email
+# BAMBUDDY_OIDC_REQUIRE_EMAIL_VERIFIED=true
+# BAMBUDDY_OIDC_ICON_URL=
+# BAMBUDDY_OIDC_AUTOLOGIN=false
+#
+# Booleans accept true/1/yes; anything else keeps the default.
+#
+# AUTO_LINK_EXISTING binds an OIDC identity to an existing local account with
+# the same email address. With EMAIL_CLAIM=email it is refused unless
+# REQUIRE_EMAIL_VERIFIED=true, because an identity provider that does not
+# verify addresses would let anyone claim someone else's account. The whole
+# config is then skipped and logged; the app still starts.