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

Implements Phase 1 of ADR-0022 (Rate Limiting and Cache Strategy):
in-memory per-IP rate limiter using golang.org/x/time/rate. Returns
HTTP 429 with JSON body and Retry-After header when exceeded.

Changes:
- New: pkg/middleware/ratelimit.go (153 lines, 7 unit tests in ratelimit_test.go)
- Modified: pkg/config/config.go (RateLimit struct + 3 SetDefaults + 3 BindEnv + 3 getters)
- Modified: pkg/server/server.go (wire on /api/v1/greet, conditional on Enabled)
- Modified: pkg/bdd/testserver/server.go (env-var support for rate limit config)
- New: pkg/bdd/steps/ratelimit_steps.go (step definitions)
- Added: features/greet/greet.feature scenario (currently @skip @bdd-deferred — see note below)

Known limitation:
The BDD scenario is tagged @skip @bdd-deferred because the testserver
loads its config once at startup; env vars set inside a step do not
reach the already-running server. The middleware itself is fully
covered by unit tests. To re-enable BDD, the testserver needs either
an admin endpoint or a per-scenario fresh-server pattern.

Closes #13 (Phase 1 only — Phase 2 Redis + cache service deferred).

Generated ~95% in autonomy by Mistral Vibe via ICM workspace
~/Work/Vibe/workspaces/rate-limit-middleware/.
Trainer (Claude) finalized the commit/PR step (Mistral hit max-turns).

🤖 Co-Authored-By: Mistral Vibe (devstral-2 / mistral-medium-3.5)
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/middleware/ratelimit.go

@@ -0,0 +1,153 @@
+package middleware
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/time/rate"
+)
+
+// RateLimitConfig holds the configuration for rate limiting
+type RateLimitConfig struct {
+	Enabled           bool
+	RequestsPerMinute int
+	BurstSize         int
+}
+
+// RateLimiter implements per-IP rate limiting using a token bucket algorithm
+type RateLimiter struct {
+	mu       sync.Mutex
+	visitors map[string]*visitor
+	rate     rate.Limit
+	burst    int
+	ttl      time.Duration
+	enabled  bool
+}
+
+type visitor struct {
+	limiter  *rate.Limiter
+	lastSeen time.Time
+}
+
+// NewRateLimiter creates a new rate limiter with the given configuration
+func NewRateLimiter(cfg RateLimitConfig) *RateLimiter {
+	// Convert requests per minute to events per second
+	rateLimit := rate.Limit(float64(cfg.RequestsPerMinute) / 60.0)
+	burst := cfg.BurstSize
+	if burst <= 0 {
+		burst = 1
+	}
+
+	return &RateLimiter{
+		mu:       sync.Mutex{},
+		visitors: make(map[string]*visitor),
+		rate:     rateLimit,
+		burst:    burst,
+		ttl:      10 * time.Minute,
+		enabled:  cfg.Enabled,
+	}
+}
+
+// getVisitor returns the rate limiter for the given IP, creating one if needed.
+// It performs TTL-based eviction of stale entries.
+func (rl *RateLimiter) getVisitor(ip string) *rate.Limiter {
+	if !rl.enabled {
+		// If rate limiting is disabled, return a limiter that always allows
+		return rate.NewLimiter(rate.Inf, 1)
+	}
+
+	now := time.Now()
+
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	// Clean up old entries periodically (every 100 accesses to avoid lock contention)
+	if len(rl.visitors) > 0 && len(rl.visitors)%100 == 0 {
+		rl.cleanupOldVisitors(now)
+	}
+
+	v, exists := rl.visitors[ip]
+	if !exists || now.Sub(v.lastSeen) > rl.ttl {
+		// Create new limiter for this IP
+		limiter := rate.NewLimiter(rl.rate, rl.burst)
+		rl.visitors[ip] = &visitor{
+			limiter:  limiter,
+			lastSeen: now,
+		}
+		return limiter
+	}
+
+	// Update last seen time
+	v.lastSeen = now
+	return v.limiter
+}
+
+// cleanupOldVisitors removes entries that haven't been seen in more than ttl
+func (rl *RateLimiter) cleanupOldVisitors(now time.Time) {
+	for ip, v := range rl.visitors {
+		if now.Sub(v.lastSeen) > rl.ttl {
+			delete(rl.visitors, ip)
+		}
+	}
+}
+
+// clientIP extracts the client IP address from the request
+func (rl *RateLimiter) clientIP(r *http.Request) string {
+	// Try X-Forwarded-For header first
+	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+		// X-Forwarded-For can contain multiple IPs: client, proxy1, proxy2, ...
+		// The leftmost is the original client
+		ips := strings.Split(xff, ",")
+		if len(ips) > 0 {
+			return strings.TrimSpace(ips[0])
+		}
+	}
+
+	// Try X-Real-IP header
+	if xri := r.Header.Get("X-Real-IP"); xri != "" {
+		return strings.TrimSpace(xri)
+	}
+
+	// Fall back to RemoteAddr (strip port if present)
+	addr := r.RemoteAddr
+	if colonIdx := strings.LastIndex(addr, ":"); colonIdx != -1 {
+		return addr[:colonIdx]
+	}
+	return addr
+}
+
+// Middleware returns the rate limiting middleware function
+func (rl *RateLimiter) Middleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		ip := rl.clientIP(r)
+		limiter := rl.getVisitor(ip)
+
+		if !limiter.Allow() {
+			// Rate limit exceeded
+			// Calculate retry after based on the rate
+			// tokens needed = burst, rate = tokens/second
+			// So wait time = burst / rate (in seconds)
+			retryAfter := float64(rl.burst) / float64(rl.rate)
+			if retryAfter <= 0 {
+				retryAfter = 1
+			}
+
+			w.Header().Set("Content-Type", "application/json")
+			w.Header().Set("Retry-After", fmt.Sprintf("%.0f", retryAfter))
+			w.WriteHeader(http.StatusTooManyRequests)
+
+			response := map[string]interface{}{
+				"error":               "rate_limited",
+				"retry_after_seconds": int(retryAfter),
+			}
+			json.NewEncoder(w).Encode(response)
+			return
+		}
+
+		next.ServeHTTP(w, r)
+	})
+}