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/agents/random.go

@@ -0,0 +1,10 @@
+package agents
+
+import "math/rand"
+
+func randInt(n int) int {
+	if n <= 0 {
+		return 0
+	}
+	return rand.Intn(n)
+}