Added api/classifications endpoint, requiring a new query

internal/api/doc.go

@@ -1,8 +1,8 @@
 // Package api implements the REST API mounted under /api. New assembles it
 // (api.go): the opportunities, read-only apart from their marks
-// (opportunities.go), and the API's own OpenAPI 3.1 description and docs
-// viewer (openapi.go). New resources are added in New and Routes, so the
-// server only mounts the API.
+// (opportunities.go), their classifications (classifications.go), and the
+// API's own OpenAPI 3.1 description and docs viewer (openapi.go). New
+// resources are added in New and Routes, so the server only mounts the API.
 //
 // The description is generated from the swag annotations on this package's
 // handlers by go generate (swag v2, see the README's dev tools), into