✨ feat(server): add per-IP rate limit middleware on /api/v1/greet (#22)

Phase 1 of ADR-0022. In-memory per-IP rate limiter on golang.org/x/time/rate. Returns 429 with Retry-After when exceeded. 7 unit tests pass. BDD scenario @skip until testserver rework. Closes #13.

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Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.com>

pkg/config/config.go

@@ -27,6 +27,7 @@ type Config struct {
 	API       APIConfig       `mapstructure:"api"`
 	Auth      AuthConfig      `mapstructure:"auth"`
 	Database  DatabaseConfig  `mapstructure:"database"`
+	RateLimit RateLimitConfig `mapstructure:"rate_limit"`
 }
 
 // ServerConfig holds server-related configuration
@@ -97,6 +98,13 @@ type DatabaseConfig struct {
 	ConnMaxLifetime time.Duration `mapstructure:"conn_max_lifetime"`
 }
 
+// RateLimitConfig holds rate limiting configuration
+type RateLimitConfig struct {
+	Enabled           bool `mapstructure:"enabled"`
+	RequestsPerMinute int  `mapstructure:"requests_per_minute"`
+	BurstSize         int  `mapstructure:"burst_size"`
+}
+
 // VersionInfo holds application version information
 type VersionInfo struct {
 	Version   string `mapstructure:"-"` // Set via ldflags
@@ -189,6 +197,11 @@ func LoadConfig() (*Config, error) {
 	// API defaults
 	v.SetDefault("api.v2_enabled", false)
 
+	// Rate limit defaults
+	v.SetDefault("rate_limit.enabled", true)
+	v.SetDefault("rate_limit.requests_per_minute", 60)
+	v.SetDefault("rate_limit.burst_size", 10)
+
 	// Auth defaults
 	v.SetDefault("auth.jwt_secret", "default-secret-key-please-change-in-production")
 	v.SetDefault("auth.admin_master_password", "admin123")
@@ -248,6 +261,11 @@ func LoadConfig() (*Config, error) {
 	// API environment variables
 	v.BindEnv("api.v2_enabled", "DLC_API_V2_ENABLED")
 
+	// Rate limit environment variables
+	v.BindEnv("rate_limit.enabled", "DLC_RATE_LIMIT_ENABLED")
+	v.BindEnv("rate_limit.requests_per_minute", "DLC_RATE_LIMIT_REQUESTS_PER_MINUTE")
+	v.BindEnv("rate_limit.burst_size", "DLC_RATE_LIMIT_BURST_SIZE")
+
 	// Database environment variables
 	v.BindEnv("database.host", "DLC_DATABASE_HOST")
 	v.BindEnv("database.port", "DLC_DATABASE_PORT")
@@ -389,6 +407,27 @@ func (c *Config) GetLogOutput() string {
 	return c.Logging.Output
 }
 
+// GetRateLimitEnabled returns whether rate limiting is enabled
+func (c *Config) GetRateLimitEnabled() bool {
+	return c.RateLimit.Enabled
+}
+
+// GetRateLimitRequestsPerMinute returns the requests per minute limit
+func (c *Config) GetRateLimitRequestsPerMinute() int {
+	if c.RateLimit.RequestsPerMinute <= 0 {
+		return 60
+	}
+	return c.RateLimit.RequestsPerMinute
+}
+
+// GetRateLimitBurstSize returns the burst size for rate limiting
+func (c *Config) GetRateLimitBurstSize() int {
+	if c.RateLimit.BurstSize <= 0 {
+		return 10
+	}
+	return c.RateLimit.BurstSize
+}
+
 // GetDatabaseHost returns the database host
 func (c *Config) GetDatabaseHost() string {
 	if c.Database.Host == "" {