feat(db): Phase 1 — DB-Schema, goose-Migrations, GORM-Models

Initialer Schema-Set (8 Migrationen, 13 Tabellen) für EdgeGuard v1:
users + audit_log + system_settings, ha_nodes, backends/domains/
routing_rules/tls_certs, forward_proxy_acls, wireguard_peers,
firewall_rules, dns_zones/dns_records, licenses. Migrations liegen
in internal/database/migrations/ (analog mail-gateway) und werden
per //go:embed ins Binary gepackt — keine separate SQL-Dateien im
.deb. ValidateMigrations + Test schützen vor Duplicate-Versionen
(mail-gateway 2026-05-08-Vorfall). GORM-Models für alle Tabellen,
sensible Felder (password_hash, private_key_enc) sind json:"-".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/models/system_setting.go

@@ -0,0 +1,11 @@
+package models
+
+import "time"
+
+type SystemSetting struct {
+	Key       string    `gorm:"column:key;primaryKey" json:"key"`
+	Value     string    `gorm:"column:value"          json:"value"`
+	UpdatedAt time.Time `gorm:"column:updated_at"     json:"updated_at"`
+}
+
+func (SystemSetting) TableName() string { return "system_settings" }