Safer defaults, read-only role, security headers and syslog TCP limits

- ALLOW_PURGE is now false by default; the UI shows a banner when there is
  no authentication.
- Read-only role: AUTH_VIEWER_USER/AUTH_VIEWER_PASS in local mode, or
  OIDC_ADMIN_GROUP in OIDC mode; changes get 403 and the admin settings
  are greyed out.
- Content-Security-Policy (inline scripts allowed by hash) and other
  security headers; cross-site changes are refused.
- Syslog TCP: at most SYSLOG_TCP_MAX_CONNS connections, closed after
  SYSLOG_TCP_IDLE of silence; HTTP idle timeout.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.env.example

@@ -9,6 +9,9 @@ AUTH_MODE=local
 # local mode: user and password (empty = no authentication)
 AUTH_USER=
 AUTH_PASS=
+# local mode: optional read-only account (can search and export, cannot change tags, sources or purge)
+AUTH_VIEWER_USER=
+AUTH_VIEWER_PASS=
 # local mode: PNG logo shown on the login page, path inside the container (empty = no logo).
 # Mount the file in docker-compose.yml, e.g. ./logo.png:/config/logo.png:ro, then LOGIN_LOGO=/config/logo.png
 LOGIN_LOGO=
@@ -28,12 +31,16 @@ OIDC_CLIENT_SECRET=
 OIDC_REDIRECT_URL=https://logs.example.org/auth/callback
 # Requested scopes (openid is always added)
 OIDC_SCOPES=openid profile email
+# oidc mode: only members of this group are admins, the others are read-only (empty = everyone is admin).
+# The groups come from the ID token claim OIDC_GROUPS_CLAIM (default groups)
+OIDC_ADMIN_GROUP=
+OIDC_GROUPS_CLAIM=groups
 # Reverse DNS: show host names instead of IP addresses (on/off)
 RDNS=on
 # DNS server used for reverse lookups (e.g. your router: 192.168.1.1). Empty = system resolver
 DNS_SERVER=
 # Allow "Delete all logs" in Settings (true/false)
-ALLOW_PURGE=true
+ALLOW_PURGE=false
 # Maximum number of rows in a CSV export
 EXPORT_MAX=100000
 # Collect the logs of the Docker containers of this machine (on/off)