refactor: modularize chat backend, update docs for .m4s segments

backend/chat/src/ratelimit/ratelimit.go

@@ -0,0 +1,75 @@
+package ratelimit
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+// RedisClient is the subset of Redis operations needed for rate limiting.
+type RedisClient interface {
+	Incr(ctx context.Context, key string) (int64, error)
+	Expire(ctx context.Context, key string, expiration time.Duration) error
+	Decr(ctx context.Context, key string) (int64, error)
+}
+
+// CheckMessageRate checks if a user has exceeded the message rate limit.
+// Returns true if allowed, false if rate limited.
+func CheckMessageRate(ctx context.Context, redis RedisClient, username string, max int, window time.Duration) (bool, error) {
+	if redis == nil {
+		return true, nil // No Redis = no rate limiting
+	}
+	key := fmt.Sprintf("rate:%s", username)
+	count, err := redis.Incr(ctx, key)
+	if err != nil {
+		return true, err // Fail open on Redis error
+	}
+	if count == 1 {
+		_ = redis.Expire(ctx, key, window)
+	}
+	return count <= int64(max), nil
+}
+
+// CheckLoginRate checks if an IP has exceeded the login attempt limit.
+// Returns true if allowed, false if rate limited.
+func CheckLoginRate(ctx context.Context, redis RedisClient, clientIP string) (bool, error) {
+	if redis == nil {
+		return true, nil
+	}
+	key := fmt.Sprintf("login_attempts:%s", clientIP)
+	count, err := redis.Incr(ctx, key)
+	if err != nil {
+		return true, err
+	}
+	if count == 1 {
+		_ = redis.Expire(ctx, key, 5*time.Minute)
+	}
+	return count <= 5, nil
+}
+
+// CheckWSConnRate checks if an IP has exceeded the WebSocket connection limit.
+// Returns true if allowed, false if rate limited.
+// Caller must call Decr on disconnect.
+func CheckWSConnRate(ctx context.Context, redis RedisClient, clientIP string) (bool, error) {
+	if redis == nil {
+		return true, nil
+	}
+	key := fmt.Sprintf("ws_conn:%s", clientIP)
+	count, err := redis.Incr(ctx, key)
+	if err != nil {
+		return true, err
+	}
+	if count == 1 {
+		_ = redis.Expire(ctx, key, 60*time.Second)
+	}
+	return count <= 5, nil
+}
+
+// DecrWSConn decrements the WebSocket connection counter for an IP.
+func DecrWSConn(ctx context.Context, redis RedisClient, clientIP string) {
+	if redis == nil {
+		return
+	}
+	key := fmt.Sprintf("ws_conn:%s", clientIP)
+	_, _ = redis.Decr(ctx, key)
+}