Add complete infrastructure automation and documentation

- infrastructure/setup.sh: Master provisioning script for fresh Ubuntu servers
- infrastructure/docker-compose.yml: Production Docker Compose configuration
- infrastructure/.env.template: Environment variables template
- infrastructure/nginx/: Nginx configuration with security headers
- infrastructure/postfix/: Postfix + OpenDKIM email configuration
- infrastructure/README.md: Complete disaster recovery guide
- scripts/docfast-backup.sh: SQLite backup script with rotation

All services now fully reproducible with documented disaster recovery procedures.

infrastructure/postfix/opendkim.conf

@@ -0,0 +1,38 @@
+# OpenDKIM Configuration for DocFast
+
+# Logging
+Syslog yes
+SyslogSuccess yes
+LogWhy yes
+
+# Operating mode (s = sign, v = verify, sv = both)
+Mode sv
+
+# Canonicalization
+Canonicalization relaxed/simple
+
+# Domain and selector
+Domain docfast.dev
+Selector mail
+KeyFile /etc/opendkim/keys/docfast.dev/mail.private
+
+# Network
+Socket inet:8891@localhost
+PidFile /run/opendkim/opendkim.pid
+
+# Security
+OversignHeaders From
+TrustAnchorFile /usr/share/dns/root.key
+UserID opendkim
+
+# Trusted hosts (who can send mail through this server)
+InternalHosts /etc/opendkim/TrustedHosts
+ExternalIgnoreList /etc/opendkim/TrustedHosts
+
+# Additional security options
+RequireSafeKeys yes
+SendReports yes
+ReportAddress "postmaster@docfast.dev"
+
+# Performance
+MaximumHeaders 30