refactor: rewrite backend in Go, replacing Python FastAPI

Port the FastAPI backend to Go 1.26, preserving all API functionality:
- auth (register/login/guest/logout/profile/history) with JWT + bcrypt
- scripts (list/detail) backed by embedded sample_data.json
- games (create/join/start/chat/phase/clue/vote/state/replay) + WebSocket
- rule-based NPC agent plus DeepSeek/Ollama AI backends
- SQLite persistence via modernc.org/sqlite (pure Go, no cgo)

Frontend and documents are unchanged.

backend/internal/config/config.go

@@ -0,0 +1,83 @@
+package config
+
+import (
+	"bufio"
+	"os"
+	"strconv"
+	"strings"
+)
+
+type Config struct {
+	DatabaseURL              string
+	SecretKey                string
+	Algorithm                string
+	AccessTokenExpireMinutes int
+
+	AIBackend      string
+	DeepSeekAPIKey string
+	DeepSeekBaseURL string
+	DeepSeekModel  string
+	OllamaBaseURL  string
+	OllamaModel    string
+}
+
+var Cfg = load()
+
+func loadDotEnv() {
+	f, err := os.Open(".env")
+	if err != nil {
+		return
+	}
+	defer f.Close()
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		idx := strings.Index(line, "=")
+		if idx <= 0 {
+			continue
+		}
+		key := strings.TrimSpace(line[:idx])
+		val := strings.Trim(strings.TrimSpace(line[idx+1:]), "\"'")
+		if _, ok := os.LookupEnv(key); !ok {
+			os.Setenv(key, val)
+		}
+	}
+}
+
+func getenv(key, def string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return def
+}
+
+func getenvInt(key string, def int) int {
+	v := os.Getenv(key)
+	if v == "" {
+		return def
+	}
+	n, err := strconv.Atoi(v)
+	if err != nil {
+		return def
+	}
+	return n
+}
+
+func load() *Config {
+	loadDotEnv()
+	return &Config{
+		DatabaseURL:              getenv("DATABASE_URL", "sqlite+aiosqlite:///./mmgame.db"),
+		SecretKey:                getenv("SECRET_KEY", "mmgame-dev-secret-key-change-in-production"),
+		Algorithm:                getenv("ALGORITHM", "HS256"),
+		AccessTokenExpireMinutes: getenvInt("ACCESS_TOKEN_EXPIRE_MINUTES", 60*24*7),
+		AIBackend:                getenv("AI_BACKEND", "rule"),
+		DeepSeekAPIKey:           getenv("DEEPSEEK_API_KEY", ""),
+		DeepSeekBaseURL:          getenv("DEEPSEEK_BASE_URL", "https://api.deepseek.com"),
+		DeepSeekModel:            getenv("DEEPSEEK_MODEL", "deepseek-chat"),
+		OllamaBaseURL:            getenv("OLLAMA_BASE_URL", "http://localhost:11434"),
+		OllamaModel:              getenv("OLLAMA_MODEL", "qwen2.5:7b"),
+	}
+}