fix(api/v2): publish OpenAPI Servers and make schemas publicly fetchable

Huma's SchemaLinkTransformer (enabled by default) emits a `$schema`
field on every JSON response and an example URL in the spec. Both were
broken in our setup: the example URL used Huma's "https://example.com"
placeholder because no Servers were declared, and the runtime URL
pointed at /schemas/Label.json instead of /api/v2/schemas/Label.json
because Huma can't see the Echo group prefix.

Two changes:

- Set OpenAPI Servers to a list with the relative GroupPrefix first and,
  if service.publicurl is configured, the absolute deployment URL
  second. Servers[0] feeds Huma's getAPIPrefix / addSchemaField /
  Transform fallback; Servers[1] is informational metadata for SDK
  generators and docs UIs. Keeping the relative URL at index 0 dodges a
  Huma quirk that double-prefixes the runtime $schema URL when the
  index-0 server URL carries a path component.

- Add /api/v2/schemas/:schema to unauthenticatedAPIPaths so editors and
  SDK tooling can fetch schemas without a token, mirroring how the spec
  itself is reachable.

pkg/routes/api/v2/huma.go

@@ -20,7 +20,9 @@ package apiv2
 import (
 	"context"
 	"net/http"
+	"strings"
 
+	"code.vikunja.io/api/pkg/config"
 	"code.vikunja.io/api/pkg/modules/humaecho5"
 	"code.vikunja.io/api/pkg/version"
 
@@ -65,6 +67,20 @@ func NewAPI(e *echo.Echo, g *echo.Group) huma.API {
 		{"JWTKeyAuth": {}},
 		{"APITokenAuth": {}},
 	}
+	// The relative entry MUST stay at index 0. Huma's SchemaLinkTransformer
+	// reads Servers[0] in three places (getAPIPrefix, addSchemaField, the
+	// runtime Transform fallback). With a path-bearing absolute URL at
+	// index 0, the runtime fallback concatenates that URL onto a ref that
+	// already includes /api/v2, producing a double-prefixed $schema link
+	// like https://host/api/v2/api/v2/schemas/Label.json. A relative URL
+	// at index 0 keeps the prefix in the transformer's bookkeeping while
+	// the absolute URL at index 1 advertises the deployment URL to SDK
+	// generators and docs UIs.
+	servers := []*huma.Server{{URL: GroupPrefix}}
+	if publicURL := strings.TrimRight(config.ServicePublicURL.GetString(), "/"); publicURL != "" {
+		servers = append(servers, &huma.Server{URL: publicURL + GroupPrefix})
+	}
+	oapi.Servers = servers
 	return api
 }
 

pkg/routes/routes.go

@@ -342,6 +342,7 @@ var unauthenticatedAPIPaths = map[string]bool{
 	"/api/v2/openapi-3.0.yaml":          true,
 	"/api/v2/docs":                      true,
 	"/api/v2/docs/scalar.standalone.js": true,
+	"/api/v2/schemas/:schema":           true,
 }
 
 // collectRoutesForAPITokens collects all routes for API token permission checking.