Add backend capacity manager

internal/capacity/client.go

@@ -0,0 +1,40 @@
+package capacity
+
+import (
+	"context"
+
+	"gitea.maximumdirect.net/eric/promptkit/internal/domain"
+	"gitea.maximumdirect.net/eric/promptkit/internal/llm"
+)
+
+type client struct {
+	manager *Manager
+	next    llm.Client
+}
+
+// NewClient wraps next with configured active-generation limits. A nil manager
+// leaves next unchanged.
+func NewClient(manager *Manager, next llm.Client) llm.Client {
+	if manager == nil {
+		return next
+	}
+	return &client{
+		manager: manager,
+		next:    next,
+	}
+}
+
+func (c *client) Generate(
+	ctx context.Context,
+	req domain.GenerateRequest,
+) (*domain.GenerateResponse, error) {
+	pool := c.manager.getPool(req.Target.BackendID)
+	if pool == nil {
+		return c.next.Generate(ctx, req)
+	}
+	if err := pool.acquire(ctx); err != nil {
+		return nil, err
+	}
+	defer pool.releaseActive()
+	return c.next.Generate(ctx, req)
+}