Harden the edge: catch-all server, modern TLS and HSTS

- Unknown hostnames and direct IP access no longer land on the first
  vhost (Gitea): port 80 closes the connection, except for ACME
  challenges, and port 443 refuses the handshake.
- Mozilla intermediate ciphers (forward secrecy only, no CBC), session
  cache, no session tickets, and HSTS with a short max-age for now.
- Set X-Forwarded-For to the client address instead of appending to
  whatever the client sent.

edge/conf.d/01-default.conf

@@ -0,0 +1,25 @@
+# Catch-all for Host headers no vhost claims (scanners, direct IP access,
+# stale DNS). Without it nginx hands them to the first vhost it loaded,
+# i.e. Gitea. ACME stays reachable here so a certificate for a new hostname
+# can be issued before its vhost exists.
+server {
+    listen 80 default_server;
+    listen [::]:80 default_server;
+    server_name _;
+
+    location /.well-known/acme-challenge/ {
+        root /var/www/certbot;
+    }
+
+    location / {
+        return 444;
+    }
+}
+
+# No certificate to present for an unknown name, so refuse the handshake.
+server {
+    listen 443 ssl default_server;
+    listen [::]:443 ssl default_server;
+    server_name _;
+    ssl_reject_handshake on;
+}

edge/snippets/proxy.conf

@@ -1,7 +1,9 @@
 proxy_http_version 1.1;
 proxy_set_header Host $host;
 proxy_set_header X-Real-IP $remote_addr;
-proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+# The edge is the first hop, so whatever X-Forwarded-For the client sent is
+# made up: replace it instead of appending to it.
+proxy_set_header X-Forwarded-For $remote_addr;
 proxy_set_header X-Forwarded-Proto $scheme;
 proxy_set_header Upgrade $http_upgrade;
 proxy_set_header Connection $connection_upgrade;

edge/snippets/tls.conf

@@ -1,2 +1,13 @@
+# Mozilla "intermediate" profile, without the DHE suites (they'd need a
+# dhparam file): forward secrecy on every connection, no CBC.
 ssl_protocols TLSv1.2 TLSv1.3;
-ssl_ciphers HIGH:!aNULL:!MD5;
+ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305;
+ssl_prefer_server_ciphers off;
+ssl_session_cache shared:SSL:10m;
+ssl_session_timeout 1d;
+ssl_session_tickets off;
+
+# Short max-age for now: raise it (e.g. 63072000) once every hostname is
+# confirmed working over https. A location that sets its own add_header
+# stops inheriting this one.
+add_header Strict-Transport-Security "max-age=86400" always;