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/store.go

@@ -0,0 +1,85 @@
+package config
+
+import (
+	"encoding/json"
+	"os"
+	"sync"
+)
+
+// Store persists web-UI editable runtime state (sources added/edited) to a
+// JSON file so edits survive restarts. Base YAML sources are merged underneath.
+type Store struct {
+	mu   sync.Mutex
+	path string
+	data RuntimeConfig
+}
+
+func NewStore(path string) *Store {
+	return &Store{path: path}
+}
+
+// Load reads the runtime file (missing file = empty state).
+func (s *Store) Load() error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	data, err := os.ReadFile(s.path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			s.data = RuntimeConfig{}
+			return nil
+		}
+		return err
+	}
+	return json.Unmarshal(data, &s.data)
+}
+
+// List returns the runtime sources (those edited via web UI).
+func (s *Store) List() []Source {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	out := make([]Source, len(s.data.Sources))
+	copy(out, s.data.Sources)
+	return out
+}
+
+// Upsert adds or replaces a runtime source and persists.
+func (s *Store) Upsert(src Source) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for i := range s.data.Sources {
+		if s.data.Sources[i].Name == src.Name {
+			s.data.Sources[i] = src
+			return s.persistLocked()
+		}
+	}
+	s.data.Sources = append(s.data.Sources, src)
+	return s.persistLocked()
+}
+
+// Remove deletes a runtime source and persists.
+func (s *Store) Remove(name string) (bool, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	kept := s.data.Sources[:0]
+	removed := false
+	for _, src := range s.data.Sources {
+		if src.Name == name {
+			removed = true
+			continue
+		}
+		kept = append(kept, src)
+	}
+	if !removed {
+		return false, nil
+	}
+	s.data.Sources = kept
+	return true, s.persistLocked()
+}
+
+func (s *Store) persistLocked() error {
+	b, err := json.MarshalIndent(s.data, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(s.path, b, 0644)
+}