feat(services): multi-domain, DNS check, maintenance + basic-auth gates (C2)

service_domains table holds the full hostname set; services.domain stays
the derived default and is synced on every domain write. Runtime spec
carries all domains into a multi-Host Traefik rule plus access gates.

- GET|POST /services/:id/domains, DELETE + POST /:domain_id/default
- GET /services/:id/domains/check — DNS preflight vs public_ip/base_url
  (ok | wrong-target | pending), stamps last_checked_at
- maintenance_mode → Traefik redirectregex to /api/v1/maintenance
  (base_url/BASE_URL setting; falls back to noop@internal offline)
- basic_auth [{username,password}] → bcrypt htpasswd on the label;
  responses return usernames only
- First domain auto-defaults; domain="" via PUT clears all domains
- CLI: services domains list|add|remove|default|check, update
  --maintenance/--basic-auth; MCP: 5 domain tools + update args
- Settings UI: Domains card (CRUD + DNS check badges) + Access card
- Fixes latent C1 bug: PUT without volumes 500'd — nil []byte sent as
  bytea, not NULL; COALESCE needs untyped nil + ::jsonb cast

Verified live: domain CRUD + default sync, pending/wrong-target checks,
bcrypt htpasswd and both middlewares on the deployed container's labels.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/database/sqlcdb/models.go

@@ -202,6 +202,19 @@ type Service struct {
 	RestartPolicy   string          `json:"restart_policy"`
 	PublishedPort   int32           `json:"published_port"`
 	Volumes         json.RawMessage `json:"volumes"`
+	MaintenanceMode bool            `json:"maintenance_mode"`
+	BasicAuthUsers  string          `json:"basic_auth_users"`
+}
+
+type ServiceDomain struct {
+	ID            uuid.UUID    `json:"id"`
+	ServiceID     uuid.UUID    `json:"service_id"`
+	Domain        string       `json:"domain"`
+	IsDefault     bool         `json:"is_default"`
+	CertType      string       `json:"cert_type"`
+	CertStatus    string       `json:"cert_status"`
+	LastCheckedAt sql.NullTime `json:"last_checked_at"`
+	CreatedAt     time.Time    `json:"created_at"`
 }
 
 type ServiceTemplate struct {