feat: add per-IP rate limiting to login endpoint

Add a token-bucket rate limiter (golang.org/x/time/rate) that limits
login attempts per client IP on POST /api/v1/login. Returns 429 Too
Many Requests with a Retry-After header when the limit is exceeded.

Configurable via LOGIN_RATE_LIMIT (requests/sec, default 1) and
LOGIN_RATE_BURST (burst size, default 5). Stale per-IP entries are
automatically cleaned up every 10 minutes.

Only the login endpoint is rate-limited per sneak's instruction —
session creation and registration use hashcash proof-of-work instead.

internal/config/config.go

@@ -48,6 +48,8 @@ type Config struct {
 	HashcashBits       int
 	OperName           string
 	OperPassword       string
+	LoginRateLimit     float64
+	LoginRateBurst     int
 	params             *Params
 	log                *slog.Logger
 }
@@ -82,6 +84,8 @@ func New(
 	viper.SetDefault("NEOIRC_HASHCASH_BITS", "20")
 	viper.SetDefault("NEOIRC_OPER_NAME", "")
 	viper.SetDefault("NEOIRC_OPER_PASSWORD", "")
+	viper.SetDefault("LOGIN_RATE_LIMIT", "1")
+	viper.SetDefault("LOGIN_RATE_BURST", "5")
 
 	err := viper.ReadInConfig()
 	if err != nil {
@@ -110,6 +114,8 @@ func New(
 		HashcashBits:       viper.GetInt("NEOIRC_HASHCASH_BITS"),
 		OperName:           viper.GetString("NEOIRC_OPER_NAME"),
 		OperPassword:       viper.GetString("NEOIRC_OPER_PASSWORD"),
+		LoginRateLimit:     viper.GetFloat64("LOGIN_RATE_LIMIT"),
+		LoginRateBurst:     viper.GetInt("LOGIN_RATE_BURST"),
 		log:                log,
 		params:             &params,
 	}