feat(backfill): implement backfill tool for generating missing embeddings

internal/config/validate.go

@@ -81,5 +81,14 @@ func (c Config) Validate() error {
 		return fmt.Errorf("invalid config: logging.level is required")
 	}
 
+	if c.Backfill.Enabled {
+		if c.Backfill.BatchSize <= 0 {
+			return fmt.Errorf("invalid config: backfill.batch_size must be greater than zero when backfill is enabled")
+		}
+		if c.Backfill.MaxPerRun < c.Backfill.BatchSize {
+			return fmt.Errorf("invalid config: backfill.max_per_run must be >= backfill.batch_size")
+		}
+	}
+
 	return nil
 }