feat(api): API keys + Redis rate limiting + usage stats

Add an optional API-key layer to the public read-only API. Keys grant
higher per-minute rate limits and attribute usage; anonymous callers are
still allowed at a lower IP-based budget.

- migration 0008_api_key: api_key table (sha256 hash only, plaintext shown once)
- apikey pkg: key generation + hashing
- ratelimit pkg: Redis fixed-window limiter + per-key usage counters; fails open
- public API middleware: X-API-Key / Bearer auth, X-RateLimit-* headers, 429+Retry-After
- admin: issue/list/revoke keys + usage view (API + UI tab)

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

api/cmd/server/main.go

@@ -12,6 +12,7 @@ import (
 	"github.com/baicai2026-baicai/goods/api/internal/config"
 	"github.com/baicai2026-baicai/goods/api/internal/handler"
 	"github.com/baicai2026-baicai/goods/api/internal/publicweb"
+	"github.com/baicai2026-baicai/goods/api/internal/ratelimit"
 	"github.com/baicai2026-baicai/goods/api/internal/store"
 )
 
@@ -31,7 +32,12 @@ func main() {
 		log.Printf("warning: database not reachable at startup: %v", err)
 	}
 
-	h := handler.New(store.New(pool), publicweb.Dist())
+	limiter := ratelimit.New(cfg.RedisURL)
+	if !limiter.Enabled() {
+		log.Print("warning: Redis not configured; public API rate limiting disabled")
+	}
+	h := handler.New(store.New(pool), publicweb.Dist()).
+		WithRateLimit(limiter, cfg.AnonRateLimitPerMin)
 
 	srv := &http.Server{
 		Addr:              cfg.Addr,