feat(config): update fallback model handling to support multiple models

internal/config/config.go

@@ -84,20 +84,22 @@ type AIEmbeddingConfig struct {
 }
 
 type AIMetadataConfig struct {
-	Model            string  `yaml:"model"`
-	FallbackModel    string  `yaml:"fallback_model"`
-	Temperature      float64 `yaml:"temperature"`
-	LogConversations bool    `yaml:"log_conversations"`
+	Model            string   `yaml:"model"`
+	FallbackModels   []string `yaml:"fallback_models"`
+	FallbackModel    string   `yaml:"fallback_model"` // legacy single fallback
+	Temperature      float64  `yaml:"temperature"`
+	LogConversations bool     `yaml:"log_conversations"`
 }
 
 type LiteLLMConfig struct {
-	BaseURL               string            `yaml:"base_url"`
-	APIKey                string            `yaml:"api_key"`
-	UseResponsesAPI       bool              `yaml:"use_responses_api"`
-	RequestHeaders        map[string]string `yaml:"request_headers"`
-	EmbeddingModel        string            `yaml:"embedding_model"`
-	MetadataModel         string            `yaml:"metadata_model"`
-	FallbackMetadataModel string            `yaml:"fallback_metadata_model"`
+	BaseURL                string            `yaml:"base_url"`
+	APIKey                 string            `yaml:"api_key"`
+	UseResponsesAPI        bool              `yaml:"use_responses_api"`
+	RequestHeaders         map[string]string `yaml:"request_headers"`
+	EmbeddingModel         string            `yaml:"embedding_model"`
+	MetadataModel          string            `yaml:"metadata_model"`
+	FallbackMetadataModels []string          `yaml:"fallback_metadata_models"`
+	FallbackMetadataModel  string            `yaml:"fallback_metadata_model"` // legacy single fallback
 }
 
 type OllamaConfig struct {
@@ -148,3 +150,45 @@ type BackfillConfig struct {
 	MaxPerRun       int           `yaml:"max_per_run"`
 	IncludeArchived bool          `yaml:"include_archived"`
 }
+
+func (c AIMetadataConfig) EffectiveFallbackModels() []string {
+	models := make([]string, 0, len(c.FallbackModels)+1)
+	for _, model := range c.FallbackModels {
+		if model != "" {
+			models = append(models, model)
+		}
+	}
+	if c.FallbackModel != "" {
+		models = append(models, c.FallbackModel)
+	}
+	return dedupeNonEmpty(models)
+}
+
+func (c LiteLLMConfig) EffectiveFallbackMetadataModels() []string {
+	models := make([]string, 0, len(c.FallbackMetadataModels)+1)
+	for _, model := range c.FallbackMetadataModels {
+		if model != "" {
+			models = append(models, model)
+		}
+	}
+	if c.FallbackMetadataModel != "" {
+		models = append(models, c.FallbackMetadataModel)
+	}
+	return dedupeNonEmpty(models)
+}
+
+func dedupeNonEmpty(values []string) []string {
+	seen := make(map[string]struct{}, len(values))
+	out := make([]string, 0, len(values))
+	for _, value := range values {
+		if value == "" {
+			continue
+		}
+		if _, ok := seen[value]; ok {
+			continue
+		}
+		seen[value] = struct{}{}
+		out = append(out, value)
+	}
+	return out
+}