feat(arch): unify frontend and backend into single Go backend server with static export

- Configure Next.js static export (output: 'export') with client-side dynamic fetching
- Implement Go static serving with SPA fallback in backend/internal/router/router.go
- Unify multi-stage build in backend/Dockerfile (Node -> Go -> minimal Alpine runtime)
- Simplify root docker-compose.yml to single anl-app service on port 8080
- Update REQUIREMENTS.md DM-03 and AC-17 normative contracts to v3.0 Unified Go Backend
- Restore strict regression verification in Gate 9 (AC-35/36/37) and unittest suite
- All Go unit tests, TypeScript type checks, ESLint, and E2E gates passed clean

backend/internal/router/router.go

@@ -3,6 +3,9 @@ package router
 import (
 	"database/sql"
 	"net/http"
+	"os"
+	"path/filepath"
+	"strings"
 
 	"git.godopu.com/lab/landing_page/backend/internal/handlers"
 	"git.godopu.com/lab/landing_page/backend/internal/httpx"
@@ -127,5 +130,54 @@ func SetupRouter(db *sql.DB, adminToken string, corsOrigins ...string) *gin.Engi
 		admin.DELETE("/standard-documents/:id", standardsHandler.DeleteStandardDocument)
 	}
 
+	// 308 Permanent Redirect for /intro -> / (H-11 / IA-02)
+	r.GET("/intro", func(c *gin.Context) {
+		c.Redirect(http.StatusPermanentRedirect, "/")
+	})
+
+	// Static assets serving
+	r.Static("/_next", "./web/_next")
+	r.StaticFile("/favicon.ico", "./web/favicon.ico")
+	r.StaticFile("/robots.txt", "./web/robots.txt")
+	r.StaticFile("/sitemap.xml", "./web/sitemap.xml")
+	r.StaticFile("/icon.svg", "./web/icon.svg")
+	r.Static("/fonts", "./web/fonts")
+
+	// NoRoute fallback for static exported HTML & 404
+	r.NoRoute(func(c *gin.Context) {
+		path := c.Request.URL.Path
+
+		// Never let a mistyped/unknown API path fall through to HTML.
+		if strings.HasPrefix(path, "/api/") {
+			httpx.NotFound(c, "Route not found")
+			return
+		}
+
+		// Try an exact static file (e.g. an asset with an extension)
+		candidate := filepath.Join("./web", filepath.Clean(path))
+		if info, err := os.Stat(candidate); err == nil && !info.IsDir() {
+			c.File(candidate)
+			return
+		}
+
+		// Try <path>/index.html — how every known page route is emitted by trailingSlash
+		indexCandidate := filepath.Join("./web", filepath.Clean(path), "index.html")
+		if _, err := os.Stat(indexCandidate); err == nil {
+			c.File(indexCandidate)
+			return
+		}
+
+		// Genuinely unknown route: serve site's 404 page with 404 status
+		notFoundPath := "./web/404/index.html"
+		if _, err := os.Stat(notFoundPath); err != nil {
+			notFoundPath = "./web/404.html"
+		}
+		if notFoundBytes, err := os.ReadFile(notFoundPath); err == nil {
+			c.Data(http.StatusNotFound, "text/html; charset=utf-8", notFoundBytes)
+		} else {
+			c.String(http.StatusNotFound, "404 page not found")
+		}
+	})
+
 	return r
 }