Rewrite backend in Go with JSON API and JS frontend

Reimplements the Flask app as a Go HTTP API (chi + modernc sqlite) with a
minimal vanilla-JS frontend in web/. Endpoints mirror the original Flask
routes but return JSON instead of HTML.

- auth: login/logout/register/sessioninfo/headerbar with crypto/rand tokens
- entry: paginated feed (single JOIN), create with image scaling
- user: profile, userinfo; delete still a stub
- requireAuth middleware passes uid via context
- notes/api.md documents the API and schema

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

main.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"encoding/json"
+	"log"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/go-chi/chi/v5/middleware"
+)
+
+func main() {
+	if err := initDB(); err != nil {
+		log.Fatalf("db init: %v", err)
+	}
+	defer db.Close()
+
+	r := chi.NewRouter()
+	r.Use(middleware.Logger)
+	r.Use(middleware.Recoverer)
+
+	// --- Auth (öffentlich) ---
+	r.Post("/auth/login", handleLogin)
+	r.Get("/auth/logout", handleLogout)
+	r.Post("/auth/newuser", handleNewUser)
+	r.Get("/auth/sessioninfo", handleSessionInfo)
+	r.Get("/auth/headerbar", handleHeaderbar)
+
+	// --- Entries ---
+	r.Get("/entry/feed/{page}", handleFeed)
+	r.Get("/entry/{pid}/interactions/{mode}", handleInteractions)
+
+	// --- User (öffentlich) ---
+	r.Get("/u/{username}", handleUserPage)
+	r.Get("/user/delete", handleUserDelete)
+
+	// --- Geschützt (Session erforderlich) ---
+	r.Group(func(r chi.Router) {
+		r.Use(requireAuth)
+		r.Post("/entry/create", handleCreateEntry)
+		r.Get("/user/info", handleUserInfo)
+	})
+
+	// Hochgeladene Medien + alte statische Assets
+	r.Handle("/static/*", http.StripPrefix("/static/", http.FileServer(http.Dir("static"))))
+
+	// Neues JS-Frontend
+	r.Handle("/*", http.FileServer(http.Dir("web")))
+
+	const addr = ":8080"
+	log.Printf("kver listening on %s", addr)
+	log.Fatal(http.ListenAndServe(addr, r))
+}
+
+func writeJSON(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(v)
+}
+
+func writeError(w http.ResponseWriter, status int, msg string) {
+	writeJSON(w, status, map[string]string{"error": msg})
+}