feat: add provider interface and registry

internal/provider/mistral.go

@@ -0,0 +1,41 @@
+package provider
+
+// MistralProvider implements Provider for Mistral services (transcription only)
+type MistralProvider struct{}
+
+func (p *MistralProvider) Name() string {
+	return "mistral"
+}
+
+func (p *MistralProvider) RequiresAPIKey() bool {
+	return true
+}
+
+func (p *MistralProvider) ValidateAPIKey(key string) bool {
+	// Mistral API keys don't have a consistent prefix, just check non-empty
+	return len(key) > 0
+}
+
+func (p *MistralProvider) SupportsTranscription() bool {
+	return true
+}
+
+func (p *MistralProvider) SupportsLLM() bool {
+	return false
+}
+
+func (p *MistralProvider) DefaultTranscriptionModel() string {
+	return "voxtral-mini-latest"
+}
+
+func (p *MistralProvider) DefaultLLMModel() string {
+	return ""
+}
+
+func (p *MistralProvider) TranscriptionModels() []string {
+	return []string{"voxtral-mini-latest", "voxtral-mini-2507"}
+}
+
+func (p *MistralProvider) LLMModels() []string {
+	return nil
+}