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/backend.go

@@ -0,0 +1,17 @@
+package models
+
+import "time"
+
+type Backend struct {
+	ID              int64     `gorm:"primaryKey"                 json:"id"`
+	Name            string    `gorm:"column:name;uniqueIndex"    json:"name"`
+	Scheme          string    `gorm:"column:scheme"              json:"scheme"`
+	Address         string    `gorm:"column:address"             json:"address"`
+	Port            int       `gorm:"column:port"                json:"port"`
+	HealthCheckPath *string   `gorm:"column:health_check_path"   json:"health_check_path,omitempty"`
+	Active          bool      `gorm:"column:active"              json:"active"`
+	CreatedAt       time.Time `gorm:"column:created_at"          json:"created_at"`
+	UpdatedAt       time.Time `gorm:"column:updated_at"          json:"updated_at"`
+}
+
+func (Backend) TableName() string { return "backends" }