feat(api): tiered cumulative quota + self-service registration

Anonymous callers get a free cumulative quota (1000 calls per IP); once
exhausted they get 403 quota_exhausted and must register. Public users can
self-register (email+password) to obtain a higher-quota API key, view usage,
and regenerate the key. Quota counters live in Redis; the public API stays
read-only except for the registration writes.

- migration 0011: app_user table + api_key.quota_total + 'registered' tier
- ratelimit: IncrTotal/TotalUsed/CopyTotal lifetime counters
- middleware: enforce cumulative quota + X-Quota-* headers
- store: RegisterUser/Authenticate/RegenerateKey (bcrypt)
- handlers: POST /api/v1/register, /account, /account/regenerate
- admin: quota_total column + registered tier
- public: 'API 密钥' account page + API docs quota section

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

api/cmd/server/main.go

@@ -37,7 +37,8 @@ func main() {
 		log.Print("warning: Redis not configured; public API rate limiting disabled")
 	}
 	h := handler.New(store.New(pool), publicweb.Dist()).
-		WithRateLimit(limiter, cfg.AnonRateLimitPerMin)
+		WithRateLimit(limiter, cfg.AnonRateLimitPerMin).
+		WithQuotas(cfg.AnonTotalQuota, cfg.RegisteredRateLimitPerMin, cfg.RegisteredQuotaTotal)
 
 	srv := &http.Server{
 		Addr:              cfg.Addr,