feat: migrate receiver to Fuego framework with OpenAPI generation

Replace net/http handlers with Fuego framework for automatic OpenAPI 3.0
spec generation. Add generated Go client package, OpenAPI extraction
script, and update Makefile with separate build/run targets for both
binaries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/extract-openapi/main.go

@@ -0,0 +1,64 @@
+//go:build ignore
+
+// ABOUTME: Extracts OpenAPI spec from Fuego server without running it.
+// ABOUTME: Run with: go run scripts/extract-openapi/main.go
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+
+	"github.com/getkin/kin-openapi/openapi3"
+	"github.com/go-fuego/fuego"
+
+	"edp.buildth.ing/DevFW-CICD/forgejo-runner-sizer/internal/receiver"
+)
+
+func main() {
+	// Create a minimal handler (store is nil, won't be used)
+	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
+	handler := receiver.NewHandler(nil, logger, "dummy", "dummy", 0)
+
+	// Create Fuego server with OpenAPI config
+	s := fuego.NewServer(
+		fuego.WithoutStartupMessages(),
+		fuego.WithEngineOptions(
+			fuego.WithOpenAPIConfig(fuego.OpenAPIConfig{
+				DisableLocalSave: true,
+				Info: &openapi3.Info{
+					Title:       "Forgejo Runner Resource Collector API",
+					Version:     "1.0.0",
+					Description: "HTTP service that receives and stores CI/CD resource metrics from collectors, providing query and sizing recommendation APIs.",
+					Contact: &openapi3.Contact{
+						Name: "API Support",
+						URL:  "https://edp.buildth.ing/DevFW-CICD/forgejo-runner-sizer",
+					},
+					License: &openapi3.License{
+						Name: "Apache 2.0",
+						URL:  "http://www.apache.org/licenses/LICENSE-2.0.html",
+					},
+				},
+			}),
+		),
+	)
+
+	// Register routes to populate OpenAPI spec
+	handler.RegisterRoutes(s)
+
+	// Output OpenAPI spec as JSON
+	spec, err := json.MarshalIndent(s.OpenAPI.Description(), "", "  ")
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error marshaling OpenAPI spec: %v\n", err)
+		os.Exit(1)
+	}
+
+	if err := os.WriteFile("docs/openapi.json", spec, 0644); err != nil {
+		fmt.Fprintf(os.Stderr, "Error writing docs/openapi.json: %v\n", err)
+		os.Exit(1)
+	}
+
+	fmt.Println("Generated docs/openapi.json")
+}