feat: ModelRouter — unified OpenAI-compatible multi-source LLM gateway

- Lua adapters per upstream (transform_request/response/stream_chunk, build_headers signing hooks)
- AUTO priority routing with per-model kind (chat/image), explicit source/model routing
- Per-source concurrency caps with queueing, exponential backoff, AUTO failover
- OpenAI-compatible API: chat completions, SSE streaming, image generations, models
- Gateway key auth, web UI for adapter/source management, runtime persistence
- e2e test running the real binary against mocked upstreams

internal/config/config.go

@@ -0,0 +1,124 @@
+// Package config loads the gateway YAML configuration plus a runtime overlay
+// (web UI edits) and resolves them into sources with per-model priority.
+package config
+
+import (
+	"fmt"
+	"os"
+	"time"
+
+	"gopkg.in/yaml.v3"
+)
+
+// Config is the top-level gateway configuration.
+type Config struct {
+	Listen        string   `yaml:"listen"`
+	GatewayKeys   []string `yaml:"gateway_keys"`
+	DefaultModel  string   `yaml:"default_model"` // e.g. "AUTO" or a model id
+	AdapterDir    string   `yaml:"adapter_dir"`
+	RuntimeFile   string   `yaml:"runtime_file"`
+	MaxConcurrent int      `yaml:"max_concurrent"` // global inflight cap, 0 = unlimited
+	Sources       []Source `yaml:"sources"`
+}
+
+// Model is a single exposed model id bound to a source, with priority used by
+// AUTO model selection (higher number = preferred).
+type Model struct {
+	ID       string                 `yaml:"id"`
+	Priority int                    `yaml:"priority"`
+	Kind     string                 `yaml:"kind"` // "chat" (default) | "image"
+	Meta     map[string]interface{} `yaml:"meta"`
+}
+
+// Source describes a single upstream LLM provider.
+type Source struct {
+	Name          string            `yaml:"name"`
+	BaseURL       string            `yaml:"base_url"`
+	APIKey        string            `yaml:"api_key"`
+	Adapter       string            `yaml:"adapter"`
+	Endpoint      string            `yaml:"endpoint"`       // chat endpoint override
+	ImageEndpoint string            `yaml:"image_endpoint"` // image endpoint override
+	Models        []Model           `yaml:"models"`
+	Headers       map[string]string `yaml:"headers"`
+	Meta          map[string]interface{} `yaml:"meta"`
+	Temperature   float64           `yaml:"temperature"`
+	MaxTokens     int               `yaml:"max_tokens"`
+	Timeout       time.Duration     `yaml:"timeout"`
+	MaxConcurrent int               `yaml:"max_concurrent"` // per-source inflight cap
+	QueueTimeout  time.Duration     `yaml:"queue_timeout"`  // wait for slot before failing
+}
+
+// Load reads and validates a config file.
+func Load(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	var cfg Config
+	if err := yaml.Unmarshal(data, &cfg); err != nil {
+		return nil, fmt.Errorf("parse config: %w", err)
+	}
+	if err := cfg.ApplyDefaults(); err != nil {
+		return nil, err
+	}
+	return &cfg, nil
+}
+
+// ApplyDefaults sets missing values and validates the config.
+func (c *Config) ApplyDefaults() error {
+	if c.Listen == "" {
+		c.Listen = ":8080"
+	}
+	if c.AdapterDir == "" {
+		c.AdapterDir = "adapters"
+	}
+	if c.RuntimeFile == "" {
+		c.RuntimeFile = "runtime.json"
+	}
+	if c.DefaultModel == "" {
+		c.DefaultModel = "AUTO"
+	}
+	seen := map[string]bool{}
+	modelOwners := map[string]string{}
+	for i := range c.Sources {
+		s := &c.Sources[i]
+		if s.Name == "" {
+			return fmt.Errorf("config: sources[%d] missing name", i)
+		}
+		if s.BaseURL == "" {
+			return fmt.Errorf("config: source %s missing base_url", s.Name)
+		}
+		if s.Adapter == "" {
+			s.Adapter = "openai"
+		}
+		if s.Timeout == 0 {
+			s.Timeout = 120 * time.Second
+		}
+		if s.QueueTimeout == 0 {
+			s.QueueTimeout = 60 * time.Second
+		}
+		if s.MaxConcurrent == 0 {
+			s.MaxConcurrent = 8
+		}
+		if seen[s.Name] {
+			return fmt.Errorf("config: duplicate source name %q", s.Name)
+		}
+		seen[s.Name] = true
+		for j := range s.Models {
+			m := &s.Models[j]
+			if m.ID == "" {
+				return fmt.Errorf("config: source %s has a model without id", s.Name)
+			}
+			if owner, ok := modelOwners[m.ID]; ok {
+				return fmt.Errorf("config: model %q defined by both %s and %s", m.ID, owner, s.Name)
+			}
+			modelOwners[m.ID] = s.Name
+		}
+	}
+	return nil
+}
+
+// RuntimeConfig is the persisted web-UI editable slice (sources added/edited).
+type RuntimeConfig struct {
+	Sources []Source `json:"sources"`
+}