feat(infra): commit fail2ban jail for /api/auth/login

Adds two files mirroring the on-host install layout:

  infra/fail2ban/filter.d/familienarchiv-auth.conf
  infra/fail2ban/jail.d/familienarchiv.conf

Filter parses the JSON access log emitted by Caddy (previous commit) and
matches 401 responses on /api/auth/login. Jail bans the offending IP for
30 min after 10 attempts in a 10-minute window.

Verified the failregex against four sample log lines via fail2ban-regex
in an alpine container:
  - 2 brute-force 401 attempts        → matched (ban)
  - 1 successful login (POST /api/auth/login 200) → not matched
  - 1 unrelated GET /login 200        → not matched
Date template "ts":{EPOCH} parses Caddy's Unix-epoch ts field.

The previous review iteration described this jail in DEPLOYMENT.md prose
only; committing it makes the security posture reproducible from a
fresh server build.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

infra/fail2ban/filter.d/familienarchiv-auth.conf

@@ -0,0 +1,29 @@
+# fail2ban filter for credential-stuffing attempts against the
+# Familienarchiv login endpoint.
+#
+# Parses Caddy JSON access log entries (configured in
+# infra/caddy/Caddyfile via the (access_log) snippet).
+#
+# Sample matched line (whitespace inserted for readability):
+#   {"level":"info","ts":1700000000.12,"logger":"http.log.access",
+#    "msg":"handled request",
+#    "request":{"remote_ip":"203.0.113.42","method":"POST",
+#               "host":"archiv.raddatz.cloud",
+#               "uri":"/api/auth/login",…},
+#    "status":401,…}
+#
+# Caddy emits remote_ip *inside* the request object and status at the
+# top level. The order within the request object is stable
+# (remote_ip → … → uri) across Caddy 2.7+. Lazy `.*?` keeps the regex
+# robust to header-dict size growth.
+
+[INCLUDES]
+before = common.conf
+
+[Definition]
+failregex = ^\s*\{.*?"remote_ip":"<HOST>".*?"uri":"/api/auth/login.*?"status":\s*401\b
+
+ignoreregex =
+
+# Caddy's ts field is a Unix epoch with sub-second precision.
+datepattern = "ts":{EPOCH}

infra/fail2ban/jail.d/familienarchiv.conf

@@ -0,0 +1,27 @@
+# Jail definition for the Familienarchiv login endpoint.
+#
+# Install: ln -sf /opt/familienarchiv/infra/fail2ban/jail.d/familienarchiv.conf \
+#                 /etc/fail2ban/jail.d/familienarchiv.conf
+#          ln -sf /opt/familienarchiv/infra/fail2ban/filter.d/familienarchiv-auth.conf \
+#                 /etc/fail2ban/filter.d/familienarchiv-auth.conf
+#          systemctl reload fail2ban
+#
+# Verify with:
+#   fail2ban-client status familienarchiv-auth
+#   fail2ban-regex /var/log/caddy/access.log familienarchiv-auth
+#
+# Tuning rationale:
+#   - maxretry 10: legitimate users mistyping passwords don't trip the jail
+#   - findtime 10m: rolling window that catches automated brute force
+#   - bantime 30m: long enough to discourage scripted attacks, short
+#                  enough that a user who fat-fingered their VPN comes
+#                  back online within a coffee break
+
+[familienarchiv-auth]
+enabled  = true
+filter   = familienarchiv-auth
+logpath  = /var/log/caddy/access.log
+maxretry = 10
+findtime = 10m
+bantime  = 30m
+action   = iptables-multiport[name=familienarchiv-auth, port="http,https"]