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/database/database.go

@@ -0,0 +1,118 @@
+package database
+
+import (
+	"database/sql"
+	"strings"
+
+	_ "modernc.org/sqlite"
+
+	"mmgame/internal/config"
+)
+
+var DB *sql.DB
+
+func normalizeDSN(url string) string {
+	if url == "" {
+		return "mmgame.db"
+	}
+	for _, prefix := range []string{"sqlite+aiosqlite:///", "sqlite:///"} {
+		if strings.HasPrefix(url, prefix) {
+			path := strings.TrimPrefix(url, prefix)
+			if path == ":memory:" {
+				return "file::memory:?cache=shared"
+			}
+			return path
+		}
+	}
+	return url
+}
+
+func Init() error {
+	dsn := normalizeDSN(config.Cfg.DatabaseURL)
+	db, err := sql.Open("sqlite", dsn)
+	if err != nil {
+		return err
+	}
+	db.SetMaxOpenConns(1)
+	if err := db.Ping(); err != nil {
+		return err
+	}
+	DB = db
+	return createTables()
+}
+
+func createTables() error {
+	stmts := []string{
+		`CREATE TABLE IF NOT EXISTS users (
+			id VARCHAR(36) PRIMARY KEY,
+			email VARCHAR(255) UNIQUE,
+			nickname VARCHAR(50),
+			avatar_url TEXT,
+			password_hash VARCHAR(255),
+			role VARCHAR(20) DEFAULT 'player',
+			is_guest BOOLEAN DEFAULT 0,
+			game_count INTEGER DEFAULT 0,
+			token_version INTEGER DEFAULT 0,
+			created_at TEXT
+		)`,
+		`CREATE TABLE IF NOT EXISTS game_sessions (
+			id VARCHAR(36) PRIMARY KEY,
+			script_id VARCHAR(36),
+			status VARCHAR(20) DEFAULT 'waiting',
+			phase VARCHAR(100) DEFAULT '',
+			phase_index INTEGER DEFAULT 0,
+			started_at TEXT,
+			completed_at TEXT,
+			config TEXT,
+			created_at TEXT
+		)`,
+		`CREATE TABLE IF NOT EXISTS session_players (
+			id VARCHAR(36) PRIMARY KEY,
+			session_id VARCHAR(36),
+			user_id VARCHAR(36),
+			role_id VARCHAR(100),
+			role_name VARCHAR(100) DEFAULT '',
+			is_human BOOLEAN DEFAULT 1,
+			is_ready BOOLEAN DEFAULT 0,
+			avatar_url TEXT,
+			status VARCHAR(20) DEFAULT 'active',
+			joined_at TEXT
+		)`,
+		`CREATE TABLE IF NOT EXISTS chat_messages (
+			id VARCHAR(36) PRIMARY KEY,
+			session_id VARCHAR(36),
+			sender_role_id VARCHAR(100),
+			sender_name VARCHAR(100) DEFAULT '',
+			message_type VARCHAR(20) DEFAULT 'public',
+			content TEXT,
+			target_role_id VARCHAR(100),
+			clue_id VARCHAR(100),
+			phase_index INTEGER DEFAULT 0,
+			created_at TEXT
+		)`,
+		`CREATE TABLE IF NOT EXISTS clues_state (
+			id VARCHAR(36) PRIMARY KEY,
+			session_id VARCHAR(36),
+			clue_id VARCHAR(100),
+			status VARCHAR(20) DEFAULT 'unreleased',
+			revealed_by VARCHAR(100),
+			released_at TEXT,
+			created_at TEXT
+		)`,
+		`CREATE TABLE IF NOT EXISTS votes (
+			id VARCHAR(36) PRIMARY KEY,
+			session_id VARCHAR(36),
+			round INTEGER DEFAULT 1,
+			voter_id VARCHAR(36),
+			target_id VARCHAR(36),
+			reason TEXT,
+			created_at TEXT
+		)`,
+	}
+	for _, s := range stmts {
+		if _, err := DB.Exec(s); err != nil {
+			return err
+		}
+	}
+	return nil
+}